Is this an option to add another external power jack using the same electrical configuration as the onboard jack (7-15V)?
I wonder about that too. I think the X2 power connection is the same as the main barrel plug but without the reverse polarity protection diode. There is some info on this post but it's about an older version of the axoloti board that has only 2 solder pads for X2.
yeah, that was my issue, and Im a bit hesitant to use it and potentially fry the board.
That is true yes. My Solution was to put wires from the + - of the dc jack to some of the unused pins on the bottom of your picture. Works perfectly fine.
Update for anyone finding this later...I attached a barrel jack to these inputs and they work great with a 9V power supply.