Either of those suggestions would work. The Yamaha device is a little pricey for what you need, but would allow you to get even more inventive, e.g. controlling modulation instead of volume.
 
If you go with the separate conversion box, you'll have the flexibility to use just about any expression pedal, which could be a plus if the pedal breaks. And they do break down. I just got a new Yamaha FC-7 yesterday to replace a busted M-Audio pedal, and I'm impressed with its apparent quality and heft. A great deal for $40.