Slaves can never be occupants, at least not in the way you might be thinking. If you enslave them, you damn them to eternal slavery, forever tainted by whoever decides to use them next, a life of servitude until they lie in a broken heap, all because you decided it'd be fun to put a collar on them. You monster.
Occupants on the other hand~ Just talk to a mugger when you get the chance, it'll start bringing up their friendship. At a high enough level it'll tell you that you have the option to take them home.