If you mod the Casse unit pool, you'll have the Casse tech tree and government system and so on. I guess it depends whether you want to be Sauromatae in Ireland, or Casse with horse archers in England...

FWIW, I've done migrations both ways (modding files and using cheat codes). I found using the cheat codes is a lot easier, not being a modder before I started. It took me all of ten minutes to move the KH to Massalia. More to the point, it's far harder to go wrong if you do things via the user interface instead of modding a set of documentation-challenged data files with unfamiliar cross-dependencies.