Hi!
You *can* control the overall volume of all MultiPads together (this is done via the mixer). Having said that, the mixer does *not* allow the control of the individual volume of each of the four MultiPads.
But, since the MultiPad file is actually a specialized MIDI file, which can be edited on the keyboard itself, you could add volume-control messages to the MPAD file to control individual volumes. Not as fast and simple as using the mixer, but it's a possibility.
Regards!