[PvK]

One thing you can do is mod the units used by existing independents who are limited to certain terrain types, though you'll need to gather that info of which indy poptypes (ones you'd want to replace) are limited to which terrains. Or you might be able to mod existing recruiting sites and their units and then modify maps so that there are such sites in appropriate locations, which would be more detailed but more work and limited to specific maps. And of course, sites would be the way to add them all to the capital. Either of those approaches result in anyone being able to recruit those trolls from those locations when they control them, of course, though you could make the discovery requirements match national mages that the troll nation has.

Another option would be to simply make them all recruitable everywhere, and then specify rules which players are supposed to follow unless they want to cheat...