Adding a unit to a faction through the game.

[zerathule]

You need the unpacker to fully add a unit to a faction roster.
But you can bypass this by adding these lines in the export_descr_unit :
with the Mercenary Arquebusiers description put instead of this line :

ownership slave

this line :

ownership slave, scotland

You'll be able to hire them in custom battle.
Now if you want them in campaign map add this in the export_descr_building :
Lets say you want to add these in the last level of the city barracks add this line :

huge_stone_wall city requires factions { northern_european, mesoamerican, middle_eastern, eastern_european, greek, southern_european, }
{
capability
{
recruit_pool "Conquistadores" 1 0.5 4 1 requires factions { spain, portugal, } and hidden_resource america

add this :

huge_stone_wall city requires factions { northern_european, mesoamerican, middle_eastern, eastern_european, greek, southern_european, }
{
capability
{
recruit_pool "Conquistadores" 1 0.5 4 1 requires factions { spain, portugal, } and hidden_resource america
recruit_pool "Mercenary Arquebusiers" 1 0.5 4 1 requires factions { scotland, } and hidden_resource america

Hope it helps :)