This is a massive topic, and there might be at least two questions here, aside from the combination-specific advice you seek:
"How do I implement effective kicks into close-range combos?", and
"How do I maintain sufficient distance to kick 'normally'?".
I will outline some ideas which will enable you to do further, more specific research.
Kicking at Close Range
Develop good mobility (range of movement in joints), particularly in the hips, but also knees.
Develop good flexibility (the ability of the muscles to stretch), particularly in the lower back, gluteals and hamstrings.
Mobility and flexibility enable you to add more kicks to your close-range repertoire (as per point 3). This in turn makes your attacks less predictable and much harder to defend. Crescent kicks, axe kicks, high (down-kicking) roundhouses, sharp upward roundhouses to the floating rib, even tight front-kicks and leg kicks are all potentially very effective at close range.
Increase your kicking repertoire (the number of kicking techniques available to you). Aside from the kicks above, remember the value of inside leg kicks. They are often under-utilised, are relatively easy to execute, require little energy and can be performed quickly without much commitment. Verhoeven understands this, but remember it can be executed off both legs and from close and long ranges. Also develop specific close-range round-house
Train for close-range (such as when doing heavy-bag work, shadow-boxing and sparring). This sounds obvious, but it is very rare (at least in my experience) to see fighters train the techniques mentioned above with any regularity. You tend to fight as you train, so your training should reflect the diversity of techniques you aspire to use in fights. Don't train these elements as an afterthought either, or as an option, but regularly, with the understanding that many people don't train to defend against them, so the reward to effort ratio is potentially higher than for standard techniques.
Develop legal clinching techniques which diminish your opponent's ability to read and react to your lower-body movement.
This is a big topic in itself. Some points to remember:
Discourage your opponent from closing range (unless you want them to), by drawing upon fast jab-like teeps, inner-leg counters to their standing leg when moving/kicking, and of course, with a nice, long, fast jabs to the face. Remember, once you've developed an arsenal of close range techniques, you might want your opponent to get nice and close.
Utilise fast lateral and backwards movement.
Take the initiative every now and then. Instead of countering all the time, put your opponent under forward pressure. Make them retreat. It is easier for you to close in than it is to counter or retreat. I'm not advocating a relentlessly forward style at all, just that you remember the advantages to be gained by being unpredictable, which includes being aggressive on occasion.
Incorporate intense pushing-based rounds into your heavy-bag work. Try this: Find one of the heavier bags in your gym (within your strength ability), and try to maintain position in the spot where the bag would usually hang. In other words, your task in this exercise is to keep the bag away from you, even though it is constantly wanting to fall back into you and topple you over. Combine pushes with a range of strikes, and you will see that this drill will improve your fitness quite rapidly. It is not trained enough and replicates the stresses of some fight situations uniquely well (in the absence of a partner).
Jab, Cross, Hook, Right RoundHouse kick
Cross, Hook, Cross, Left Switch Roundhouse
Watch a range of elite fighting videos and record how often you see these combinations being implemented. If you wish and/or your coach want you to persevere with them as stepping stones towards a library of combinations, be aware that its easy to fall into some bad habits when training them.
For example, your jab and/or cross might be very effective, causing your opponent to move back or laterally, in which case your hook will require at least one or two chase steps to have any chance of landing. Your roundhouse kick might need to be long or very short. If you constantly train this combo in the same way, on the same bag, it will be of limited use in fight contexts, in which each of your techniques will often produce different opponent reactions every time you execute them. You need to train on bags which hardly move at all, bags which swing like mad, and ideally with pad/sparring partners who move unpredictably and force you to adjust on the fly.
Training combinations is important, but (perhaps obviously) a combination becomes far more effective if it exists within a wide range of combinations. It might be argued that, whilst certain well-selected combinations - especially if unexpected - can prove very effective against certain fighters, even when relied upon almost to the exclusion of other techniques, the 'ideal' fighter, at least in my mind, is the fighter who transcends combinations and enters a flow state in which almost any technique can follow from any other. This is admittedly a very high standard to aspire to; one that will take years and years of smart, consistent, structured and cumulative work, but it is achievable if you constantly challenge yourself to train outside your comfort zone.
One problem with overtraining combinations is that you drill yourself into a habit of throwing an entire sequence without thought and without consideration of each opponent's unique reaction(s) to your attacks. This is partly why lots of well-protected, medium-contact sparring is essential; because you learn this rather quickly in the ring and come to understand how vital adaptation is in a fight.
You might want to try training your combinations first by emphasising each strike equally, so that they begin to flow. You reap power from fluidity. Once each technique 'fits' as part of the whole, experiment with emphasising different strikes. For example, if you want to finish on a long, powerful roundhouse, you might want to throw a quick but light and long 'ghost' hook to raise the opponent's gloves whilst avoiding forward commitment and maintaining range. Alternatively, you could step aggressively in with a nice tight short hook and throw it very hard, following up with a close-range, chopping roundhouse to mid thigh.
The switch roundhouse is useful for range control because it can be used to burst forward, remain neutral, or move back. If you are coming off a right cross, remember to practice fully rotating your hips, so that you maximise the potential range of your cross (keeping your chin tucked behind your right shoulder). Many people only rotate half way and lose range here.
You probably feel as though I'm not addressing your specific combos enough. I'm trying to avoid being too prescriptive here, and to emphasise tactical thought, as there are simply far too many permutations to go through.
Is there any advice for going from punches to kicks in a combo for
Much of what is written above is applicable to this, but general training principles to concentrate on here would be:
Start thinking about your initial punches as a means to set up following strikes (either immediately or with strategic intent).
Start thinking about whether what you now consider as 'optimal range' is actually optimal. If you become comfortable fighting at all ranges (perhaps via the methods outlined above), then you become a much more capable fighter; a fighter able to comfortably and effectively adjust to different techniques, styles and contexts. Most fighters in my experience don't do this, so you'll be at an advantage if you take the steps recommended here. It takes time of course, but also reinforces the notion of training 'smart' and efficiently as opposed to merely 'hard'.
Remember not every punch needs to be thrown with near maximum force. If you want an opponent to stay in range after a series of jabs for example, it makes sense for these jabs to be lighter, used a means primarily of raising the hands and obscuring vision to enable more effective kicks.
If an opponent is at the end of jab range, they are in perfect position for many kicks, including teeps and roundhouses so the jab provides a nice gauge. The teep provides a slightly greater 'defensive' range if you want to stay a little further away, whether it be for rest, to deter an aggressive fighter or to control the timing of your attacks.
Very few opponents are used to receiving kicks near-simultaneously with punches, as Verhoeven has demonstrated. When we fight and train, we get into rhythms of sorts, and any disruption of this rhythm can prove to be quite useful. So, to follow from the above example, if you throw your jab out a few times then, on the final jab - sacrificing any real power, try an inside leg kick (off the same or opposite side). There is a very good chance that if you are successful here, your opponent's posture will fail and you will for a moment have good access to their head.