Below you can find the two examples that I suggested as a reference for implementing the requested feature. I hope that they will be of good use to you.
The first one shows how to create a RadDock object on the client by initially dragging a fixed dock inside a zone that acts as a toolbox. Please note that there is no server side interaction - the docks are created on the client only. You can modify it further to suit your specific scenario.
The second example shows how to create RadDock objects by having the initial drag come from a RadTreeView control. This example creates the dock controls on the server-side and adds them to the specific zone.
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