Set single modifier press as trigger?

Hi, I discovered a neat function in QMK (keyboard programming language) that turns a single modifier press into a custom keystroke or combo. I would like to use BTT to implement something like this on any keyboard. For example, single press on left alt to switch to the previous app, and single press on right alt to switch to the next app. Or single press on left shift for page up and left control for page down. This adds a ton of one-touch functionality to any keyboard and I cannot figure out how to do it with BTT. It just won't accept a single modifier as a trigger. Am I missing something, or is this feature not available? Thanks!