Good idea, but unfortunately, no
I believe you can for organ type patches:
According to the MIDI Guide page 12:
Volume control
CC#11: Expression [bn, 0B, vv]
Adjusts the volume of the program. This controls the volume on the output line for both the KEYBOARDS and ENSEMBLE sections.
Note: On the CX-3 type organ programs, this controls the volume before the rotary speaker input. Recreates the effect of an expression pedal used with drawbar organs. The expression pedal effect can be recreated in the same way for VOX organ and electric organ programs as well.
Note:This functions differently from normal in some programs. For programs that include the words 'Pedal Wah', the pedal controls the pedal wah effect within the program, not the volume. When the 'Foot - Pedal' function in system settings is set to 'Expression (CC#11)' and the pedal is operated, an expression message will be transmitted, and the volume will change. When an expression message is received by another device, a similar effect to the one that is applied when operating the pedal on the Grandstage will apply.
The pedal should be configured to send CC 11 by default but if not you can set it under system settings (p 15 of the user manual)
I was hoping it could be done for EP's, since there is no way to adjust "amp drive" level on the panel (I don't think) other than through "edit" menus, or (on/off) S1 switch.
(BTW, I don't own a Grandstage, but thinking of buying the 73key. The manual is kind of confusing (to me) on this issue, and no stores near me have ever had them on demo.)