I have Humax YouView with 500 Gb of Internal Hard Drive in England and I have Humax FreeSat in France with 500Gb External (USB) Hard Drive, I record programmes on both which I can watch later, here come the question, would the recordings be compatible with one another, i.e. the YouView recordings can be watched on the Freesat and vice versa?
From your initial question and what you are asking, and not asking, and not said, I suspect you haven't begun to think your question through for yourself.
If you extract the recording and can't play them using something like VLC or MPHC on a PC then they won't play on anything else.
You are not helping yourself very much by not clearly stating the model you have in France. You mention that your 'freesat' in France has an external drive - if the 'freesat' box it is an HB-1000S recordings are encrypted and are only playable by the HB-1000S that recorded them.
You don't say how you have extracted the youview recordings, but are already asking about compatibility! As all youview HD recordings are on an encrypted partition have you cracked the encryption? To extract the SD recordings you will either need to remove the hard drive each time to extract the recordings or you could consider modifying the youview box so that the SATA cable comes out of the box and is connected to an esata/esata cable with the other end of the esata/esata cable going back into the box using a second esata/sata cable. The recordings are held on an XFS partition and so you will need to be OK accessing that on a computer.
Regarding going the other way with your mystery box's recordings playing on the youview box have you managed to get anything other than youview's own recordings to play on the youview box? No one has ever reported how to easily, and practically, do that on a youview box.
What you are trying to do has a few different hurdles to surmount. I suggest the same as Black Hole. It won't be a question of a specific place to look but on a lot of general reading to get up to speed so that you can come up with a solution, or a tentative solution to confirm or discuss, and hopefully report back on how you achieved it and what you would have done differently if starting again.
Or are you just asking about putting the two units on the same local network and using DLNA?