Look for a file called descr_win_conditions.txt (in the mod's folder, if it has one). It's pretty self-explanatory. Assuming XGM installs into its own folder it'll be in XGM\data\world\maps\campaign\barbarian_invasion or imperial_campaign

You can either do something like this (verify region name in descr_regions.txt):
Code:
hold_regions Latium
take_regions 50
or like what vanilla has:
Code:
take_rome
take_regions 50