Starting a tool window as a tab

I have a TreeView, DockManager and a single tool window. As the TreeView's nodes are selected the contents of the tool window are changed. I have the option where the user can right click on a tab and say "Open in new tab" (similar to Firefox) and a new tab is supppose to open. The user can then dock the tabs where ever he wants. If the user selects "Open in a new tab" again, it will open the tab in the current ToolWindow.

My question is, how do I have the new tool window be a tabbed window of the current ToolWindow? I want it to be automatically tabbed when it opens. Similar to dragging two tool windows on top of each other.