On adding a new transfer, tixati behaves like the files don't exist yet (which is true for most users).
Unfortunately, there is no option to add transfers from existing files *and an existing .torrent file* - creating a new .torrent is not an issue.

It would be great if you could add

* an option to easily create transfers from partially downloaded files and an existing .torrent file. While is possible via setting all files to "off", then manually copy existing files into the download location, then do a force-check; it is not very straightforward.

* an option to create transfers from finished downloads and an existing .torrent file *without requiring a read-write filesystem* for seeding existing torrents from read-only file sources. Choosing a download location is currently not possible if the location folder is in a read-only mounted filesystem ("OK" button is grayed).

I've tested this with tixati_1.99-1_amd64.deb.