What you are looking for is under Settings > Download Clients > Torrent Blackhole
This helps create a "blackhole" folder that is now monitored for any completed files. Radar or Sonarr will then try and import these files if the naming matches anything in your library that's missing.
So in theory you can manually add torrents to your client which you can categorize in such a way that they move to this blackhole folder on completion.
Obviously the folder or category can be called anything, but I just call them blackhole_movies, blackhole_series.