We're in Maryland, and my Dad has two outdoor ponds and the goldfish winter over in the ponds just fine.
How deep is the pond? When I was helping my Dad build the ponds we made sure that they were at least 20 inches deep so that there would still be liquid water at the bottom. We also float a pond heater.
Goldfish in the winter here almost go dormant without the heater. With the heater they are a little more active. It's funny watching them swim under the ice.
If we've had success wintering the fish over here in MD then I would think you'll be fine in NC.
Jonathan