Enabling units for different factions

[Stark]

Can I enable recruitment of Parthian Armored Horse Archers for Arche Seleukiea?

I would like to roleplay the creation of units similar to byzantine kataphraktoi, capable for both melee combat and long range archery, and those "Shivatîr-î Zrêhbârân" seem like the closest thing in EB (if there are some better units for my intended purpose please share).

I would be very thankfull if some kind soul could explain me how to do this. I've never modded RTW, but I have some minor experience from other games, so I hope I'll understand quickly.

P.S. If possible I would also like to rename them and up their secondary attack a bit.