Hey Brian,
yes, but the Masso offers spindle speed override by itself. And the Masso is also capable of feedrate override, one of the missing and most requested features for the buildbotics-based Onefinity Standard Series Controller.
Furthermore on any VFD on which you control spindle speed via analog voltage, you could at any time put a switch into the analog spindle speed line and switch to a potentiometer for temporary manual speed control.