The thing to remember here is that distortion is often compressing the volume too. This means that while a volume pedal after your distortion will definitely change the volume through your amp, one before the volume pedal will change the amount of volume going in to the distortion pedal (known amongst sound technicians as 'gain'). Often increasing the volume into a distortion pedal will not significantly effect the output volume, but drive the distortion harder, which is what you're asking for here.
Try it out, you might be surprised what you can achieve. Also, you can create this effect with your guitars volume knob, if you just want to give it a go without buying a volume pedal.
Again, distortion compresses the volume, so volume before distortion doesn't mean more volume, it means more distortion.
There are numerous effect pedals (usually higher-end) which take a control signal from the expression pedal to control specific parameters of the effect. This is quite expensive, and not really necessary if you use the technique above.