ACD features aren’t really helpful with only two agents, so you wouldn’t be missing much if you change it.
You could easily fix this using extension lists, but it would ring the first two phones simultaneously. Put the first two phones in an extension list, and make that list the first member of the hunt group. Create a second extension list with the original phones, plus the additional phones. Make this list the second member of the hunt group. In fact, make it a member several times.
Incoming calls will ring the first two phones, until the No Answer Advance timer expires, then ring on the second extension list. If the original phones are busy or in DND, calls will immediately advance to the second list. Because the original phones are also included in the second list, they can answer the call if they become available.
Putting the second extension list as a hunt group member multiple times will keep all phones ringing, instead of have the call bounce between extension lists as the No Answer Advance timer kicks in.