If worse comes to worse, you can create an interface and adapter pair. You would change all uses of ConcreteClass to use the interface instead, and always pass the adapter instead of the concrete class in production code.
The adapter implements the interface, so the mock can also implement the interface.
It's more scaffolding than just making a method virtual or just adding an interface, but if you don't have access to the source for the concrete class it can get you out of a bind.