As far as I can tell, outline mode is simply a way to have checkbox items in DT, i.e., a to-do checklist that lends itself to a GTD workflow. Yes, you can export the items in opml and then make them into an outline in something like Notebook or Outliner, but you can do the same with any group in DTP. Importing from Notebook or Outliner simply gives you a list of checkbox items in DTP. I don’t know if this is the intent, but that seems to be the case.
I do a lot of outlining in DTP (but almost never use checkboxes)
You have to switch the file list order to unsorted (not posible in 3-pane view) then you can drag and arrange documents. It is not as comfortable as in Omni Outliner but you have some DTP-benefits instead (e.g. wiki links, replica, search possibilities, multiple windows).