There is not an easy way to mod the pedal to add an expression pedal. but…. Depending on what MIX mode you are set at, and how your effect loop is set-up, and what you want to fade in/fade out… you can put a volume pedal in front of the FROM FX input to get this expression pedal fade in/out feature.
for instance, if want to pass the dry thru (DRY+FX) and have all your fx 100% wet, then you can easily mix the fx into the dry signal with a volume pedal. To fade between DRY and FX (MIX) you can probably use a stereo volume/pan pedal (ernie ball or boss) to make this work. I have not tried this, but I will investigate and make an app note for this feature if it works.