I have an old Ernie Ball VP Jr volume pedal that I had refitted with a 100K linear taper pot for use as an expression pedal. I wasn't using it any more, so I wired the pedal with a 1/8" female stereo/TRS phone jack. Now I can use a standard 1/8" stereo phone cable to connect the pedal to the SS. After using the calibration in the SS software, it works great.
FYI, the use of "volume pedal" and "expression pedal" get tossed around loosely (too much pot?) And sometimes expression pedals are wired to work with specific pieces of equipment. Some pedals use a linear pot, some use an audio/log taper pot. The SoftStep is pretty accommodating, given its calibration method, but I'm guessing it still requires a 3-wire connection. Most guitar pedal jacks just have 2-wire jacks (mono).
If you have an old geetar volume or wah pedal you wanna try, you should still be able to use it with the SS, but you will probably need to rewire the pedal's pot like I did: wire one of the outside pot connectors to the ground/sleeve of a stereo 1/8" female jack. Then wire the center connector of the pot to the center contact (ring) of the jack. Next, wire the remaining pot contact to the tip connection of the jack. Cable it up and try it with the SS, but be sure to calibrate (check the manual on how). If the pedal works backward for the way you want to use it, just swap your ground and tip connections.