I have a similar question.
Due to tweaking how my storage folder was mounted (CIFS/SMB vs. NFS) and user/group mappings, there were several recordings that failed to record this past weekend because Tvheadend reported that it could not create the file. Ultimately, I will probably download these files and add them to my Kodi library. However, I would like to add them to Tvheadend.
Is it possible if I were to transcode the files into MPEG-TS containers and name them the same as Tvheadend would have, and then restart Tvheadend, would Tvheadend find the new files move the recordings from "Failed Recordings" into "Completed Recordings"?
I know that Tvheadend has problems adding extra files to the DVR database or otherwise modifying the DVR recordings, but I'm thinking this might be possible. (I was able to do this same process to replace a corrupted/interrupted recording with a downloaded file that was transcoded into MPEG-TS and renamed the same as Tvheadend expected, and it was picked up by TVH on its next restart, so perhaps this isn't so crazy ...)
EDIT/UPDATE:
I was able to remux H.264/AAC MKV files into MPEG-TS containers, rename them appropriately and place them where Tvheadend would expect them. Then, I told those failed recordings to be moved to finished; at first Tvheadend complained saying the file was missing, but after restarting Tvheadend, it rescanned the folders, found the new file and now the previously 'failed' recording is now showing up in Tvheadend as if it had recorded them itself.