You could assign the drive parameter to the expression pedal so that in the toe possition it is full drive and at heel possition it is a low drive. Another way to do this is add a split before the drive block and join it after. Then assign the volume(s) of the two branches of the merge point to the expression pedal. Have the branch with the drive be normal and the volume of the dry branch be reversed so that as you push the toe down its volume goes lower and the drive branch goes higher.