I haven't used transmission in years but in theory it should be possible if the file is exact match. Other option would be to create a new torrent.

If you copy and paste the magnet link into transmission or any other client or will start to download from scratch. Exit the app and COPY your version of the iso into the location your bittorrent app is trying to save to. Don't move you copy, if could get damaged in the next step. Reopen the app and get it to recheck. It will analyse the file you just copied over and if exact match will allow you to start seeding. Sometimes though it doesn't work, and you end up with a 99.9% ish complete and it will attempt to redownload that part. Hence why I said make sure to COPY.