To jump in once again, you'd really need a volume or expression pedal to accomplish what you are doing. A regular sustain pedal will change everything from low speed to high speed instantaneously. The Expression pedal would change things gradually.

You could theoretically assign this same control to a slider - and it would work, but that would be pretty impractical, but then again, a lot of keyboards work this way.

I hope this helped.

The Infamous EPU.