Trying this right now and this is so ****ing annoying, like a chore. I greatly miss Warband troop editor.
Bannerlord requires manual xml tweaking and a full item ID list at hand (and there is only one in the net, which seems to be from old BL versions with some items missing). Also, every single change must be tested in custom battle. It took me 3 days of work and testing to change Imperials only. I've been stuck on Vlandia for a week now.