Welcome to the forum! Start simple and build on that when 'simple' doesn't get the job done.

Here is the outline of the 'simple' way to be able to use a foot controller to control performance volume the way you want. Please try this method first so you start getting a feel for how things work together on the instrument:

I've plugged in my expression pedal to Foot Controller 1,

1. Ok - but for this simple method plug the pedal into the Foot Controller 2 jack instead. If you view the graphic for the 'Edit - Common/audio->Common->Control Number' screen on p.167 of the Modx reference doc you will see 'Foot Ctrl 1' assigned to CC 1 and 'Foot Ctrl 2' assigned to Super Knob. We are going to use the Super Knob functionality as part of the simple method. Note: you could assign 'Super Knob' to 'Foot Ctrl 1' if you wanted but since Ctrl 2 is already assigned we will use it.

2. If you now move the pedal you will see the lights of the Super Knob change from Min to Max as the Pedal changes from its Min to Max.

3. Edit your performance and navigate to the 'Edit - Common/Audio->General->Common screen as shown on p.159

Is there a way to change the range of CC 11 to never reach 0 volume?

4. Set the performance volume to the minimum you want it to go to when the pedal is at Min

5. Make a Control Assignment to use the Super Knob to control the performance volume - remember the pedal is going to be controlling the super knob which then controls the volume.

5A. navigate to the 'Edit - Common/Audio->Control->Control Assign' screen as shown on p.166.

5B. Press 'Auto Select' and then move the Super Knob. This will change the 'Source' parameter to the Super Knob.

5C. Select the first available destination and find/select the performance Volume parameter

You should now be able to use the pedal to control the volume from the Min you set in step 4 to the Max.

Makes perfect sense, thank you!

Is there a way to change the range of CC 11 to never reach 0 volume?

No.  The range of output is between 0 (min) and CC7's setting (max).   There's not a standard for putting a minimum on this range and there's not a way to offset the volume if CC11 is 0.   This comes, more or less, from standard MIDI interpretation. 

It would be nice if there could be a CC11 offset such that you could raise the floor.  Or, alternatively, not alter CC11 at all and just have a way to setup curves on the foot controller inputs either globally or locally (per Part) such that you could force the range of the foot controller to be limited by some amount and/or follow a given shape (linear, exponential, etc).

What's been presented previously is a valid workaround where you stop using CC11 and instead use the FC input as a standard source controller for the control matrix and setup source/destination pairings through your own curves that takes care of this.

Ideascale is a place where you can suggest some other way of handling this not (yet) available.