Firstly, I am assuming you mean an expression pedal capable of outputting MIDI CC messages, correct?
The thing with MIDI CC messages is that they can only be between 0 and 127 for a grand total of 128 steps. This low resolution gets us into some trouble if you are looking for exactly 1 value, and not something close to that value.
For example (Shiroverb):
-The ratio parameter goes from 0.5 to 2, so we have a range of 1.5
-1.5 / 128 steps gives us 0,01171875 (the value change per CC)
-We want to achieve the -oct, unison, +fifth and +oct positions, which CC value represents these positions?
-The easiest are -oct and +oct, since they are on the outer ends of the dial (CC 0 and CC 127 respectively).
-unison pitch is achieved by setting the dial at 1/3 (starting point + 0.5), this should be CC 41.667 which does not exist
-+fifth is achieved by setting the dial at 2/3 (starting point + 1), this should be CC 84.333 which does not exist either
If you are not too picky about the exact value it is probably the easiest to assemble a chain like in the screenshot below, translating the CC into a note, discarding all values that you are not interested in, and transforming it back to a CC.
Hope that helps you out a bit, you can always get creative with different MIDI filters, although it will require some thinking ahead usually. If you find a better solution, be sure to let us know!