Unfortunately this functionality is not supported by the RadDock control and there no easy, out of the box, solution.
The RadDock and the RadDockZone are rendered as a <div> HTML elements on the page. Basically we have <div> (RadDock) within a <div> (RadDockZone). If you can make the <div>s behave as you have described there would not be a problem making the dock's behave the same way.
the Telerik team