Hi, Just to get it out of the way first off, I don't know how to mod medieval 2( I have modded other games like DOW40k). I just thought I'd throw this out there i don't know how to convert anything with this game, but the idea is -

- use the same armies and everything from the Britannia campgain as far as Welsh, Irish, and Scots(unit stats for highlander units are buffed). and add it into the vanilla campgain with the big map to make space for the new factions. It would be what I and all you other Celtic fans out there would want! So Someone be our Hero!