I am sorry to say that this functionality is not supported by the RadWindow. You can only move it via its TitleBar. Please note that even if the titlebar is disabled (VisibleTitleBar property is set to false) there is still a drag handler in the border, just below the resize handle, each is about 3px tall.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed