OK, so you’re going to be editing the NotePerformer Expression Map. In My case, I duplicated it to a new one, as you can see in the pic below. You will add a new expression map base type for every one of the Guitar Effects listed on the NotePerformer web site (Distortion, Fuzzbox, Heavy Distortion, etc.)
For each one, you need to have an appropriate “Playing Technique” which is what you will enter in the score, mapped to a “Playback Technique” which is what will trigger the expression map entry.
I just did it for all the Guitar Effects individually, and it took maybe 10 minutes. It will seem weird at first, but once you’re done it a couple hundred times you get used to it 
Some things to watch out for: always make sure you have a playing technique and a playback technique mapped to your expression map entry. In some cases, there’s already a playback technique (like, Distortion is already there) but mostly you will create new entries for both playing and playback. If you rename your NotePerformer Expression Map like I did, you need to go into your endpoint setup to and apply the Expression Map there, otherwise it won’t see your new entries.
I would walk you through it in more detail, but I’m just heading out. Lots of folks here know how to do this, so they can help too. For folks playing along, the full list of NotePerformer Guitar Effects is here:
Here’s my quickie demo file. Good luck!
NotePerformer Guitar Effects.dorico (644.5 KB)