The game will fill up the rest of your dorm automatically, yes. Roommates come and go from term to term, so you'll see a bunch of different NPC roommates if you play through 4 terms.
If you mean, can you get a specific townie? I don't know. You could take control of the household and send them to uni, that would work the same way we just discussed in the other thread. But to get a specific townie to move in as an NPC roommate, I don't know if there's a way to do that.