Well, first you have to create these units. Then you make them upgrade to a single unit. Then in module scripts, search for this:
(faction_set_slot, "fac_culture_1", slot_faction_tier_1_troop, "trp_swadian_recruit"),
After "trp_swadian_recruit" you can add more troops like "trp_swadian_whatever" (remember to put a , between the troops)
I think this should work, but i'ts untested