This isn't too hard to do actually. All you have to do in order to add Nomads to, say, Mnahattan is add the encounter called "area_nomads" to Manhattan's population zone (this is found in Data \ Universe \ Systems \ Li01 \ li01.ini) Look in one of the Unknown sytems (same directory as New York's right back there, but with "...Systems \ Ew05 \ Ew05.ini" or "...Systems \ Ew06 \ Ew06.ini" -- look in one of those 2 files for the name of this encounter and you could just copy and paste it into Manhattan's zone to get some Nomads
Sorry if you are not very good at editing, or if this is choppy and hard to undersatnd; I am out of town right now, away from my FL files, so I cannot be as helpful as I could be if i had files. Maybe someone else can help ya if this doesn't.
As for a mod, I don;t know of one for certain; maybe just look around, ask, or just wait for an answer in here.
"Do you see the slightest evidence anywhere out there that man was the climax towards which creation had been straining from the beginning? That creation came to a screeching halt with the birth of man?"
- Ishmael