I would strongly suggest referencing this stream here in stackoverflow:
How to configure the system menu of a Windows form?
The above thread uses unmanaged C ++, but they wrapped it pretty well, it should still work adequately in WPF.
Then you just make sure that when you click on a special option, it switches the top property of the application:
bool tpMost = false;
This.Topmost = !tpMost;