Very neat ideas, indeed!
As it is now, population and city-growth and -building in general does not feel very lively at all. By introducing some sort of "independent but recruitable" population this sad part of the game could be greatly enhanced.
In my opinion this needs quite a lot more detailed thinking, though. You start suggesting game mechanics (you even mention exact numbers) without this actually having been thought through, if I may say so. If this feature is going to be integrated into the game, I suggest we should take the time to develop the idea from general to specific. I fear though, as you have already mentioned in your first post, that this might be rather hard and time-consuming to program and integrate into the game.
I suggest we start with the general lore of the game. We are in a post-apocalyptic world. So let's gather ideas of how and where we can meet people that live in this world and could be persuaded to join our kingdom/empire. On the top of my head I can think of the following:
a) People who have already settled down in some way or another (yet did not form anything you would call a settlement /village / township)
I mean think about it: Fertile land is supposed to be scarce in this desolate world. How come this fertile land is actually unclaimed? I understand the wilds are a dangerous place, yet we repeatedly hear in the fluff texts of recruitable champions, that families own farms,so obviously there must be people who have settled down in arable locations. If unable to convince the living populace to join you but you still need the land, you would have to kill or drive off the people who live there by force.
Other settled down people could be:
alchemists / miners / hermits / a cloister / a refugee camp
b ) People who roam the wilds and make a living off of the wilds
Generally I would embrace the idea that any person could be persuaded to join you, even bandits.
This would however introduce new problems (if you can convince a robber to join
You, why wouldn't you be able to convince him to join you armed and ready to defeat your enemies?). Other people who wander the wilds and who could be a source of new population:
hunters / gatherers / herders / driven off and now homeless people / deserters / scavengers /
people who lost their occupation due to unfortunate events (mine didn't yield any resources anymore / mercenaries who weren't needed anymore / merchants who were robbed or made a critical business failure)
I'd be grateful for anyone contributing ideas on either the topic of "where and how to recruit people" or "how to proceed with fleshing out this idea in general".