If you didn't want to outright cheat, you could have your first born son tutored by an Arab and keep a Normon son as a spare. The Arab son inherits the throne, poof, youre an Arab nation and can make the empire. If you want to revert to Normon simply don't marry your Arab son and try to kill him off so the Normon son inherits. Thats the only thing I can think of.
You can often find Arabs for tutoring in your baron's courts who make excellent tutors for culture changing, or you can marry a catholic-Norman to any Muslim Arab and then re-invite her back to your court and she drags the husband along with her.
Being a Normon emperor of an Arab Empire would seem abit odd tho..