I'm afraid that RadPanelBar doesn't support drag-n-drop functionality. You can check this online example
which demonstrates treeview inside RadPanelItem template, note that its nodes can be dragged and dropped.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items