I might be wrong, so please try to verify this, but the game will create these NPC's. They aren't townies, no. If you have no mods to decrease the number of NPC the game creates, then your game should create a said number of..Clerks(teenagers), police officers, burglars, nannys etc.
The way *I* would do it, would be using SimPE and its Alt SimSurgery Plugin .
I would let the game creates them all, then look at them in SimPE, and take notes of how many of them I have. (maybe I'll find..3 teenage clecks, 3 police officers..)
Then I would create as many sims in Bodyshop, according to the gender, age of these NPC I want to replace.
Once done, I would use alt surgery to replace them by the ones I made.