A little more simple way:
in script_village_recruit_volunteers_recruit
Change this:
(party_get_slot, ":volunteer_troop", "$current_town", slot_center_volunteer_troop_type),
to this:
(store_faction_of_party, ":cur_faction", "$current_town"),
(faction_get_slot, ":volunteer_troop", ":cur_faction", slot_faction_tier_1_troop),
Then just add the troop you want the player kingdom to recruit like this is the game_start:
(faction_set_slot, "fac_players_supporter_faction", slot_faction_tier_1_troop, "troop_ID"),