I don’t think you’re really supposed to do this since the option is grayed out in Xcode when you select Editor -> Embed in Navigation Controller, but a hacky way to get around it is to control-drag from a free-standing Navigation Controller to the Tab Bar Controller and select “root view controller.”
Control-click from the Tab Bar Controller to the controller you want to be the new tab and select “view controllers.”
In Xcode, open Preferences and go to the “Key Bindings” section. Type “outline” in the search filter box. Select “Show Document Outline” and double-click to the right, in the “Key” column. A text box should appear and you can type in your new shortcut. I used Cmd+9.