Found the path where TVHeadend saves recordings information.
~/.hts/tvheadend/dvr/log
Currently a have a file named 1, which is from a recording made with TVHeadend.
Tried copying that to a file named 2, and then changing the contents to match one of my MediaPortal .TS recordings.
When restarting TVHeadend the file disappears. Nothing in the logs.
Any clues?