afaik, this is impossible. you can only confederate with your own "race"

Well thanks for the reply anyway. :) Would have been nice if someone could make a MOD. Would have made some epic rpg armies. I guess the complete extinction of all other sentient beings on the continent is the only way.

What you can do, is vassalize them or force them into allies using this: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=692076595&searchtext=

Other than that, unles I am mistaken, the confederations are hardcoded to your "race/culture". Its been this way since rome 2, where confederations debuted (in the new games).

EDIT: Also, in Attila (or was it Rome 2?), there was the option when playing the Romans to "confederate" factions outside your culture, but even if you did, all that it did was to give you their territory. You did not gain the ability to recruit their units and their cities + generals changed into the "Roman" visual style.

EDIT2: Btw, you can accomplish what you want with either the diplomacy mod. And if you dont want to rely on the AI (Which is rubbish), get unit mods. The closes that I can think of what accomplishes what you want to do, is tha AOR mods (they were very good in Medieval 2 Total War mods = especially the Third Age Total War mod, where when you played as Rohan and conquered an elven region, you could recruit elf archers, that was awesome!) AOR stands for Area of Recruitment and the idea is that if you conquer a region (for example Athel Loren), you should be able to recruit units that come from the native population of that region (in this case, wood elves).

Thanks a lot I'll look at your suggestion in the second Edit. Sounds like the best option.