If you usually open a torrent file from the browser without actually download it ( the .torrent is saved to /tmp ) the next reboot kget will, obviously, not find anymore it.
That could be a problem if the download is not ended or it's still seeding: you will get a "Torrent file doesn't exist" error and the only way to recover is to download the .torrent again and place it in the right folder (/tmp in this case).
If kget cannot keep the needed information of the torrent in its data structures, which could be one solution, another solution, IMHO, could be to save a copy of the .torrent kget is going to download somewhere ( maybe in the kget dot-dir ), before starting the download, and then refer only to that file.