Create a new user, call if FWD to Contact Center, you will find a Forwarding tab, that's where you put the number of the Contact Center. Include the leading 9 if that's what you dial before outside numbers. Tick the boxes for Forward Hunt Group Calls and Forward Internal Calls. Make a new Sequential Hunt Group, and put just this user into it. Make this new Sequential Group the Overflow for the main group. The main group itself can stay of Collective if that's what you had it set to originally, only the overflow group needs to be sequential.