I decided to move my downloaded torrent files to another computer with more space available. I wanted to still seed these files to maintain my ratio (private trackers :-)).
After some trial and error (and crosscheck with a friend) I found out it was very easy. Just upload the files to your torrent download dir (download just one file to find out which directory you must upload those files to).
Now add all the .torrent files that belong to the files you’ve just uploaded and check the do not automatically download (niet automatisch downloaden) tickbox :
After uploading, select all your files and do a force recheck:
After the force recheck, ruTorrent or your torrent client of choice sees that all the files are in place. Select start to start seeding them :