you don`t need to join a house to fight for it and maybe if a house is loosing or not paying you, you just start fighting for a different one in order to survive .
Or you even take a quest making you insult another house while actually fighting for a different one, in order to fall in their back on the battlefield…
(other ideas might be: sabotaje, raiding, taking prision( not only nobles also commoners and soldiers or even bandits(other creatures?) to sell to a house, a slave trader or a ransom breaker…)
soooooooooo many possibilities