Hi. Is it possible, on Windows, to drag-and-drop either 1 or N local files onto Roon to playback/queue?
(this would be instead of adding a music folder to permanently watch)
The answer is: ‘Yes and No’ 
You can d’n’d files into Roon, but in such case app will automatically add them to your default Organized folder, as the result they will become part of you library.
If you don’t want to see those files after the playback you need to delete them from the Organized folder.