i thought they could create them, but if they can't...

gnome editor and unitprod

find the unit name, and add FN_PAPAL to the faction association column,
if that still doesn't work
go to the build prod, and check that the papacy is able to build the right buildings for those units
finally if we are talking custom battles and not the campaign map go back to the unit prod and in the end column for the unit
mercenary association set to YES and the FN_PAPAL added to the list if not already on there.

ps, it may not be FN_PAPAL, it could be FN_PAPACY, i really can't remember check campmap/startpos/early.txt setactivefaction:: (near the top of the file) and see if it says FN_PAPAL or FN_PAPACY or something similiar, i'm sure its one of the two