Not sure what the end goal is, but if you want expression pedal for the Hammond sounds, the Electro has a control pedal jack. You may be able to patch your pedal in there and control the expression directly without bothering on the audio side of things.
This thing is a guitar pedal, so I doubt that it will work as an expression pedal. But an expression pedal is certainly the preferred option in my view, although I know some players who prefer volume pedals.
On a Hammond sound, the expression pedal should change the sound as well as the volume, wheras a volume pedal can only change the volume.
Those connectors from the first link are what you would need if you want to go with the volume pedal. Two of them - you would not need the other cable because you would be coming out into dual keyboard cables on either side, which is presumably what you use now to connect the keyboard to the amp without the pedal in circuit.
BTW, you can test if this is how it works. Plug a pair of headphones into the output of the pedal. Now plug a TS cable into the input. You should hear signal in one earpiece. Now pull the cable out slightly - you should feel it click into place again when slightly removed. You should now hear sound in the other earpiece. If this works its wired internally as I described.