By response curve, I'm talking about the way the potentiometer inside the pedal reacts as it travels from one end of its rotation to the other. Expression pedals that work like the EX-1 are relatively simply electrical devices. The position of the parameter being controlled by the expression pedal depends on the resistance seen at the expression pedal input jack.

I apologize for the technical jargon in the next paragraph, and if you don't care about the details, just skip it.

In a pedal with a linear response curve, the position of the pedal will be related roughly 1:1 with the position of the expression pedal, i.e, when the pedal it at 10% the value it's controlling will be at 10%. When the pedal is at 50%, the value will be at 50%. For many parameters, this is what you want. For volume and wah, though, most people like something different. Volume is different because are ears don't hear in a linear fashion. For something to sound twice as loud to us, it needs to have ten times the original signal strength. So this is where audio taper pots come in handy. The picture below shows some of the most common response curves:

So, if you want an expression pedal that works like a volume pedal, you'd have to find one of the volume pedals that's compatible with the M-series stuff. The thing is that you'll probably find that using a volume pedal to control other parameters doesn't work the way you'd necesarily like either. I still think that if someone wants to use a volume pedal a lot, the best solution with the M-series pedals is to get a standalone volume pedal.