I have downloaded the contents of a torrent onto my webserver. I now want to use this as a webseed; that is, I want to download the file from my webserver and from the swarm simultaneously. How can I do this?

Go to the webseeds tab and add the url.

Note that if the url ends in a / ktorrent will automatically add the name of the torrent.

I did that, and it just sat there and said "Not in use". That's with a direct link to the largest file in the torrent (several gigabytes), not the folder name.

Don't use a link to one of the files, the link needs to be to the directory where the files are in.