Yes, but that’s pretty much unrelated to YunoHost
You can find many tutorials on the Internet on how to create a torrent using Transmission, e.g. http://www.makeuseof.com/tag/create-torrent-files-share-transmission/
As for webtorrent, you can probably use it to fetch a torrent you published via transmission, as long as you get the magnet link (or the other relevant piece of information). But I don’t know the exact details on how to use webtorrent exactly…