To change an existing top level Clippboard into a subClippboard, choose your Clippboard from the menu on the left, then click the Edit button at the top of the Clippboard, and select Edit.  Use the Set as SubClippboard menu to choose the top level Clippboard you want this subClippboard to reside under.

Top level Clippboards that already have subClippboards under them cannot be changed into subClippboards. You'll need to move or delete the subClippboards before you can change the status of the top level Clippboard.