The fourth column in mercenary towns.

You can also use related factions for mercenaries, either for light-aligned factions, or generally use spirits, animals, and monsters. And Necropolis and Inferno could hire converted creatures. That is, you capture them, bring them to the city, convert them, and then place them in a queue (with irreversible replacement if there's already someone in the queue). Of course, new models would be needed. But it’s simpler here. There won’t be much need for complex solutions. Mostly just recoloring. Most of this has already been done in mods that expand neutral creatures, except perhaps for dwarves. You can use these developments.