With transmission-create you can create a torrent from the commandline. Load it into your bittorrent client, find its hash in the UI, and make it into a magnet link, like:

magnet:?xt=urn:btih:[this is where the hash goes]

Example:

magnet:?xt=urn:btih:a008663c4fc7e710bd59200e5ef99ba04f83f319