How to give unit to another faction?

I want to experiment a bit and enhance rosters for all factions. For example it would be fair to give Kuarothori and Lavotuxri for celtic factions.

I've searched the forum but couldn't find the whole process of giving unit to another faction step-by-step. Can anyone show me how to do it or give a link with a manual?

Both those units each share models with two other units the celtic factions can recriut. As the most skins a faction can have for a model is two, giving them to a celtic faction wouldn't work properly (the soldiers would appear as a different unit). You best option is to enable recruitment for the Brihentin and Solduros in the regions where the units you want are recruited. See this thread for how to do it.

Thanks, I'll see