Brian -
If your pond is below the frostline (at least 3 ft deep at the deepest point to be safe) and you keep an ice chaser (small floating heater that keeps a hole open in the ice) or a bubble system running to keep a hole open in the ice you turtles will probably Over-winter just fine. They go under and hibernate at the bottom of the pond all winter. Please be advised though that there is always the risk of losing several of them over the winter. If anything may happen to one while it is hibernating it could possibly die as you have very little chance of monitoring them once they are all down on the bottom of the pond resting the winter away.
If you keep in mind that there is that risk and you are willing to take it then as long as you have the above mentioned requirements, you can hibernate you turtles outside.