This functionality is not supported by the RadWindow control out-of-the-box.
What I would suggest you is to create an ItemsControl (for example ToolBar) with items parallel to all opened windows and to make an item visible if the RadWindow is minimized and invisible if it is not minimized. If the window is closed you should remove the Item from the ItemsControl.
Hope this helps!
the Telerik team
Browse the videos here>>
to help you get started with RadControls for WPF