Provided that I'm a CP4 owner, I did several unsuccessful attempts to control voiume of an external instrument, using CP1 master keyboard capabilities.

I connected a FC-7 Expression pedal to the corresponding assignable input, created a masterkeyboard setup with a zone assigned to the external instrument.
Although I instructed CP1 to drive external instrument's volume via FC-7 I was not able to separate that external volume to the internal CP1 Voice volume.

In a nutshell, FC-7 drove CP1 Voice and external instrument's volume at the same time.

I thinth this is an unresolved bug, quite big...