single expression pedal , dual use as volume/wah for kemper stage ?

You should be be ok (sounds like you may have done this with your midi board) ... Assign the Exp pedal as a Wah, Check the Wah>Vol box. When there is an active Wah in the Rig then the pedal will Wah! - Otherwise it will be a Volume. You will need to preset the volume before switching the wah on (takes a bit of trial and error to get the level ok but not difficult)