Crusader Kings is on the simplistic side, actually. On the scale of complexity, CK ranks dead last when compared to Paradox's other in-house titles. Here is how I would rank them, on that scale: Victoria > Hearts of Iron > Europa Universalis > Crusader Kings.
As for as translating the culture feature into Battlemaster correctly, that is impossible. Crusader Kings justifies it's culture mechanic as a means of achieving a level of realism. It succeeds at doing so, by basing the cultures on existent historical cultures. Battlemaster does not attempt realism; it takes place entirely in a fantasy setting. Therefore an attempt at realism, such as the culture mechanic, is useless.
Furthermore, cultures in Battlemaster already exist, through the form of roleplay. Aurvandil has something called "Aurvandeux" culture (forgive me if I spelled that incorrectly); Sirion has an elf culture, although it's never explicitly mentioned; and of course there are some other cultures that are often referenced, indirectly or otherwise. So an 'official' culture system would be stepping on the toes of the roleplayed culture system already in effect.