Currently, necromancer entities do the following;
-Form master-apprentice relationships, allowing towers to reproduce (so founding new sites, check).
-Launch raids against neighbouring sites (while under the player's control), presumably to eliminate local threats or just to accumulate corpses for their armies of tower-building labourers.
-Take over towns if they can't find enough zombies to build a tower.
Essentially, the only things differentiating necromancers from a civilization is the inability to trade with them and their lack of formal political positions/large scale worldgen wars against civs. Trading is not really something I can see happening with necromancers, but allowing a master of a certain number of apprentices to declare themselves Overlord over the towers constructed by their underlings and fight worldgen wars against established civs would be cool.
A better tie-in would be to just gradually dissolve the differences between civs and groups. If a group spawns multiple subordinate groups that get associated with their own sites has effectively become a civilization. I envision a scheme where all groups are associated in tributary relationships, with the name of the dominant group being the name for the overall civ. This could gradually be expanded to allow groups to secede (potentially including the player, who could then have new groups form tributary relationships with them...), new civs to be formed dynamically from bandits (or adventurer entities?) taking over sites, groups trading masters as a result of wars, dynamic civil wars as groups with enough tributaries revolt against their tributary masters, etc.