It works pretty much as you have already found out for the pedal.
Once you assign a controller to the wet/dry mix, choose an EXTERNAL # as SOURCE. These are placeholders for any type of controllers, including MIDI CCs.
Avoid using EXT 1 and 2 which are already patched to other functions by default. Try EXT #3.
Under SETUP > EXTERNAL you'll find all the EXTs#. Just locate number 3 and set it up with the CC# you want to use. This will control your wet/dry mix then, as per the first patching operation described above.
best
I
