Actually, dormies ARE townies, at least as far as the game is concerned. I made most of the Maxis-made Uni students into dormies by using Inge's teleporter bush to 'make me a townie'. The only difference is that YAs can't normally go to non-Uni lots, though Pescado's Customer Controller does let you have them come to an owned business.
If you don't have the maximum number of townies (I forget what it is -- maybe 32 of mixed ages?), the game will spawn new ones as needed. So yea, going to a comm lot could spawn new ones, and it will occasionally spawn a new one for a walk-by. If there aren't many playables in the hood, it'll spawn townies faster. Otherwise, it will take a while before it sees the need to generate a new one.
Of course, this only applies if you DON'T have notownieregen in your game.