ipsec tunnel between two fortigate

i create ipsec tunnel on fortigate Head Office to Branch both side are running fortiGate 6 os my ipsec vpn tunnel working fine both lan subnet reachable and traffic pass.. i have problem i want to add new interface on HQ fortigate for the dmz i need to reachable on branch side  for the hq DMZ interface please help me how can i do it ... thanks regards