Mocking non-virtual methods in C#

I'm trying to test some classes I've made using mocks, but I've seen all free mocking frameworks in c# aren't able to mock non-virtual methods (if it is not in an interface).

But, there's TypeMock who can do this, so it is possible. Can anyone show how to do it? I may even try to contribute to an open source framework if I can get this done.

Thanks in advance