You have two way to add new factions. First one need to know all class name of the desired faction. The second way get automatically class name for you.
_allclasse selectby the corresponding array of faction name in /core/def/mission.sqf#L639-L647.
copyToClipboard str (["EN"] call btc_fnc_get_class);.
Note: if you don’t want some class name take a look here /core/def/mission.sqf#L649-L670.