Does anyone here know if the USB/Midi port on the YPG525/625 is capable of functioniong as an 88 key controller for external gear?

I had someone ask me the other day, and I don't know anything about these newer models. The guy wants to use the YPG to control another keyboard--to give him 88 weighted keys.

Is it possible?

Thanks,
Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.