The RadWindow provides the MinimizeZoneID property which allows it to be "docked" in a certain element when it is minimized. It also has the RestrictionZoneID property which will confine its movement in the provided element. More information on the available properties can be found in this help article
As for putting a RadWindow in a RadDock - this is a rather pointless task, as both are popup controls and their content tends to be rendered as a direct child of the form element and not in the exact spot in the markup where it has been declared.
I believe I should warn you about one Silverlight issue - when it is moved in the DOM (which is what the RadDock will do when it is docked/undocked) or when styles such as overflow or display of its parent are changed (which is what the RadWindow does in some scenarios) the Silverlight app tends to restart. There is a workaround for this in RadWindow, but the case of the RadDock is more complex and you are likely to experience difficulties with it.
Generally speaking - moving a Silverlight about in the page is quite a cumbersome task and it is not always possible to achieve smooth operation.
the Telerik team