Found it,
must keep in mind the way the files/folders are structured, so you must keep that structure for torrent files:
one dir:
one sub dir (same name as the parent dir) with the files
the torent file with *.torrent extension
Copy the parent dir in
work directory of the hdd, then save the torrent file from the tracker, or other place from which you want to download, as if you would do a normal torrent download.
"update" in transmission web interface, and that's it. it starts the checkimg files process and then seeding.
The only issue is that the torrent stays in work dir and appear as active in transmission

at least for the momment, I'll wait to see if it is seeding as it should