I have seen a "pump" volume/field used in pyro sims at DOP level to make the velocity field only affect density as it passes though this pump field area (defined with geometry like a Box). The setup uses the SOP Mask field in a Volume Source node to create the mask for this "pump" area. The settings are:
Source Volume: v
Target field: vel
SOP Mask: pump

Is there a way to implement this at the SOP level using the pump "mask"?