I think it`s possible to mod it but I don`t know how.
Marry a Norman, breed, and hope to be lucky is the only advice I can give you.
There is one other. A rather complex way to do it. Change to semisalic consanguinity, marry your daughter to a Norman noble. Let them do some serious breeding, and then off your other successorsand yourself, so he inherits. The downside to this is that you must remember to switch to another county/duchy when you off your king, then save and load as your old kingdom again. The other disadvantage is that you loose your old surname, but that can be fixed by marrying your daughter to a male member of your old family.
WHO SAID BREEDING WAS EASY!!!!