Qoute from Korinov: "I'll implement a dialogue option with the minister that will allow the player to just change that and adopt a new base culture, which means the recruitable professionals and nobles and castles will be replaced by those of whatever culture you choose. I'll also give the option to revert this change (and apply it again) as many times as the player wishes, as i.e. the player may want to rely, to a certain degree, on the conquered faction's noble chaste as well (as it happened IRL)."
That option was implemented in the normal Warband, and I never knew why it is not put into the game in this mod, to choose your culture. Its a good one. However, it worked from the action menu, not with the minister.