From version 1.10 and higher you assign the expression pedal to global from the parameter "TYPE":
Press MENU, then go to PEDAL and press MENU.
Scroll to parameter "TYPE" and set it to "Exp.GlbVol". Then perform the calibration of the pedals in the following parameters.
You can use the mapping parameters (min, mid and max) to emulate a volume curve. We recommend the following values : MIN = 40%, MID = 90%, MAX =100%. Experience overall with the MID parameter so that it fits your taste - The value can surely vary from i.e. 75% to 95% depending on the pedal type.