Tvheadend

Unfortunately, I don't think so. The only way I can think of that might work would be to record some "dummy" recordings (could be just a few seconds each of any channel), copy the existing recordings that you want to import into the recordings directory, and then force move them over the "dummy" recordings (mv -f ... I think, but check the syntax for the mv command). The point is that you need to have something there that TVHeadEnd has already indexed and then you need to do a very quick operation to replace the dummy recording with yours - copy won't work because if TVHeadEnd see that the file is gone for even a few milliseconds it removes it from its index (in my experience, anyway). Once you have done that, you may be able to rename the "dummy" recordings to their original names, but that is the part I am really not certain about. I know in Kodi you can rename programs but I am not sure if it actually does it in the backend.

But there is no really good and easy way to do it that I am aware of, unless that's been added in versions after 4.0.8.

I am moving from mediaportal to tvheadend and this functionality would be fantastic if it was possible to retain my previous recordings.

I made a python script that does this. I don't know whether it is a standard option to let the MP TV Server save xml files next to the ts files, but if you have those, you can use my tool:
https://github.com/ksya/convert-mptvserver-recordings-to-tvheadend