Group Flow from FIM to AD

Hello,

Since the information normally flows from AD to FIM. I want to change the flow for Group Management, where the flow is from FIM to AD. So FIM would be in charge of adding and removing users from certain groups. When I did the switch some Users that had their Primary Group set in AD, after the switch, FIM removed that group from the "Members of" for that user, and assigned Domain Users (513) as their Primary Group. I had to go back and fix their Primary Groups, is there a way to prevent that from happening. 

Or is there a Synchronization Rule I can write up with a "Custom Expression" as for Inbound/Outbound?

Thanks