If orcs can add witch spells via FCB, I believe the common interpretation is that people that count as orcs (I.e., half orcs) can also add witch spells. Maybe you could structure it as an alternate racial trait (‘you add a number of spells equal to your strength bonus from the witch spell list to your class spell list, if any’.)
Although I'm sure there are strong combinstions, I doubt this is really any more powerful than using the Cleric list (which is already availabke to some races.
If you do allow it, they'd get access to the Cleric OR Witch list, not both (the choice is made at level 1 and cannot be changed). Giving both lists would be more powerful.