So i do this with the c76 board from cnc4pc. But on a KBIC controller. On a g0704.
Chances are high that you have a KBIC clone controller inside the spindle control box.
If so, yes you can control spindle speed this way. I chose only a forward direction but it may be doable to do both. Please advise if a KBIC controller is inside your motor box.