Is it possible to add people to a city over time and after certain events are triggered in a quest line?

If so how would I start this? Thanks in advanced!

Probably.

For the first part: my first guess would be that you'd place all your actors in the city as if you wanted them right away, and then set them as disabled so they don't show up. When you want them to appear, just change them to enabled. If you want them to appear after a certain amount of time, you'd have to keep track of time elapsed since mod enabled (or since whenever you want the timer to start). I think globals are tied to save files, so that could be used to keep track of time elapsed. Then, just enable the actors after the correct time has elapsed.

Second part: again, first thought: write a script that has a function that enables the actors you want to enable. Then, call that function from a papyrus fragment inside the appropriate quest stage.

Alright, that makes sense, thank you very much. I was hoping I wouldn't have to do something like that but I guess it makes plenty of sense.

There could be other ways of doing it. Those were just the first ideas that came to mind.