Shit, read your previous post in REVERSE. Thought you were saying you didn't mean that you wanted peds to spawn in places they wouldn't, but you wanted to know how to add them in free-roam. My mistake, anyways, I'm pretty sure this is done through MadMan's cord grabber.
There IS a tutorial on making cliques spawn in places they shouldn't (I would know as I have done it before, but it is limited to a few places) for example, spawning The Greasers on the Football field and other places (That I can't remember).... Although, I don't remember which thread it was. Best wait until someone else answers.