I looked at the manual and could not find any reference to that port. I suspect it is for running a DSC connection between the transmitter and the receiver. Here's one thing you can check to see if you can use it for a second battery connection. Check for zero Ohms continuity between the center (+) pin of that connector and the rest of the center pins, then check for the same with the outer row of pins (-), if there is continuity (zero Ohms) then you can use that port for a second battery input, regardless of its original purpose, as you won't be connecting anything to the signal pin in this case.
You can also post this in the Futaba help forum
Good luck,
Pete