Not yet. However, you can use cheats to become Muslim or pagan and then adopt their culture.
Or you can do my long legal method in the game. Swear allegeince to a Pagan or a Muslim Lord. Adopt their culture. Send your kids to be educated at the hands of Muslim or Pagan lords. Once they rule they will be the same religion as the Muslims or Pagans. This way you are playing pagan or Muslim.
To think of it, it sounds legit. Lets say you were facinated by their religion and culture and you decided to embrace it.