In theory I suppose you could but it's totally pointless; if that's all you wanted you would've been better off purchasing a standalone amplifier. As this is a multizone controller, it needs to have the zone activated, source chosen, then volume selected. Without a keypad you'd have to integrate some other form of control through the RS232 serial port.
You also must have the wall-port - as the connections go from 4x Cat5 to the wallport - and the back of the wallport is where the speakers connect and the cat5 to the other keypads.
In addition, to use an inline volume control you need the speaker wires to loop through the control location then to the amplifier; versus with this you'd wire the speakers straight to the controller's wallport and you'd wire the keypad straight there separately.
If you have the HiFi2 why didn't you just make a zone off that system versus buying a whole other multi-zone kit? Since the HiFi2 has line level outs if you don't have a free zone you could piggyback another with a separate 2-channel amp then integrate your inline volume control knob or make it a full fledged zone.
