In principle: what you do is make the recruitment of the unit dependent on the presence of another building in the settlement (say, reform_building). You make that building unconstructable anywhere - you can use a not_here hidden resource. Then you have the script add that building to every settlement once your desired conditions are met.

EB puts them in the background script, but if you trust your player you can do it through a show_me script instead - though that does limit slightly the conditions you can use.