I recently read a fan fic in which Ranma enslaved Akane against her will. While not what I consider a good fic it did inspire some thoughts. The first thing I thought was that Ranma would not enslave anyone. Upon further reflection that is a personal bias of my own. Ranma is shown to be willing to trick, manipulate, bribe, blackmail, and even use mind altering substances/magic to get people to do what he wants in the manga. It really is not that far fetched to believe he'd enslave someone. I can't really think of a reason as to why Ranma would do so though. I can't see him up and deciding to enslave someone without a reason and if he did he's extremely likely to feel guilty and let them free after a short time (Lima Syndrome). I can think of quite a few reasons as to why Ranma would kidnap someone though and I suppose that could count as enslavement, though I generally do not associate them as being the same (just like prison is enslavement). The most likely reason he'd kidnap someone is probably to get a tactical advantage (while Ranma hadn't actually kidnapped anyone in the manga, unless you count things like tying P-chan to the Charlotte cup, it does happen a number of times in the manga and Ranma is pretty pragmatic when it comes to combat).
I started to think about Ranma having a slave. The first idea that came to me is that someone that is very submissive gives themselves to Ranma; voluntarily becomes a slave to him. None of the cast members are submissive they pretty much all are fairly aggressive except for possibly Kasumi. I have no idea as to how Ranma would react if this happened, besides finding the situation to be extremely weird. How he reacts is likely to be majorly influenced by the person who wants to be enslaved and their reasons for wanting it. On his own I figure he'd view it as being a Daimyo/Samurai type relationship since that is probably the most familiar to him.
After thinking about it some more I thought why would someone enslave themselves. I can think of a lot. In most of the world for most of known history slavery existed and it was often enough better than the alternative. It was sometimes easier, better, and occasionally you had more rights/privileges than the alternative. I don't want to defend slavery since I find it to be immoral but given a choice I would rather have been a slave than an extremely poor free person depending on place and time in history. Slaves were property so they were often taken care of and had more protections than expendable poorly paid laborers. Depending on the time and place there were more laws protecting slaves than free persons (you could get away with killing a free not land owning person a lot easier than an owned slave for example).
I'm getting off topic the fic was about Ranma enslaving someone against their will. I can think of a number of ways to do this most of which I can not see Ranma doing unless the situation was extremely dire. The most likely ways I see him doing so is via accident using a magic item he doesn't really understand following that having kidnapped someone and letting Stockholm syndrome sink in. One thing from the fic I can't see happening at all is Ranma physically taking advantage of someone sexually (even in the form of kissing). Ranma places an extreme amount of importance on kissing (not at all uncommon in Japanese society where kissing is considered an extraordinarily intimate act) and has some intimacy issues (it requires more trust and openness than he's willing to give).
It is not unreasonable to think that if Ranma did not consider the being to be human he'd enslave it. After all the majority of people do not view our enslavement of animals as being morally wrong or even view it as slavery (cows, dogs, horses, etc.). What if Ranma enslaved a Kami via kotodama (also written kototama; a Japanese belief that mystical powers dwell in words and names)? Ranma somehow finds out a kami's true name and uses that to enslave the kami (this is a pretty common belief throughout the world; the ancient Egyptians for example had similar beliefs). Ranma knowing about kotodama is extremely easy to explain first he is shown to go to Shinto shrines multiple times and second in many Japanese martial arts, such as aikido which the founder of the style (Morihei Ueshiba) used kotodama as a spiritual basis for his teachings, it is considered a fundamental part of the art (using a kiai to gather inner energy or spirit is a form of kotodama). Seeing this written could be pretty interesting if for no other reason than almost every fan fic I can remember reading focuses on the physical aspects of martial arts neglecting the spiritual and religious aspects that many martial arts, particularly Japanese styles, have entirely (other than using ki blasts). I don't think I could write it though too many moral issues and a fic like this, at least if I tried to write it, would likely be too philosophical.