I was wondering, what file(s), and what would I have to modify within said file(s) to make some of the units that are recruitable only in the campaign playable in a custom battle? If I could get a file name, what I would have to modify within the files, etc. I don't want to make any other factions playable, just particular units for the already existing factions (and the Mughal Empire).

Any help is greatly appreciated!