The OpenROV code already has stubs for all axis of motion. The commands are currently
Each passes a value between -1 and 1 down to the thruster code.
The thruster files are setup so that you listen to the incoming commands and then it is up to you to covert those to the right motor commands. The code in the OpenROV repos uses a pattern of settings the requested state for each motor in a variable (new_p for port thruster) and then the actual motor value in (p for port thruster).
In the 30.0.3 release of the software, all of the commands were setup in the API to pass through the beaglebone down to the Arduino. However in the browser software, the controls were never linked up to send the Roll or Strafe commands. That is updated in the master branch of the software.