It can be done. The cheapest way is to find a " dead " evse that you can relieve of it's cord. I cam
across a dead Chevrolet Volt EVSE that I relieved of it's original cord and put another J1772 end on it
so that it was basically an extended J1772 cable. I use it when someone ICEs a charging space.
if you're looking to extend your 80amp power to the truck, you'll need some serious wiring size to make that happen. That's more-than-likely meaning that you'll need to remove the current J1772 cord 'end' from the Charging unit and add your new wiring cable there, with a junction box in-between it and your J1772 cable.