As of 9/28/16, the following Casio Electronical Musical Instruments support the use of an expression pedal:
Casio does not manufacture an expression pedal, but there are many that are compatible.
Our User's Manual properly states:
Expression pedal with a 10k ohm linear pot.
These have been tested by Tokyo as listed in the manual:
Fatar VP-25, VP-26
A very common pedal is the M-Audio EX-P, which like the pedals listed above is 10k ohms. This is a perfectly good option, but it does not function properly unless the polarity switch on the bottom is set to "M-Audio". If the switch on the bottom is set to "Other", then the pedal will not work smoothly, even if it has been "calibrated" in the System Settings of the keyboard.