It depends on what you mean by an external document. If you have a
custom UR template for a Word document, creating a new item of that template will create a stored Word document in UR. If you then edit the item's document
internally or
externally, a temporary external copy of the file will be created in the UR temp path for editing with Word (and updated in UR afterwards). The external copy will be deleted when exiting UR.
If you want the external copy of the Word document to be kept permanently in some other location on your file system, after creating the item in UR, set the item's URL attribute to the path+filename to use, then
sync the item (Item | Synchronize) to create the file in the location specified in the URL attribute. I would recommend then unstoring the item from UR (Item | Store Contents) so that editing will open this external copy instead of a temporary copy of the stored document.