I have created a little project that, although very bare-bones, allows you to load and develop your own mods for Snap!, as a extension, similar to Scratch Addons, called Snap!Mods (if you couldn’t tell from its title).
Here’s the GitHub repo, of course. A example mod, example.js, is provided for you to see how to make your own mods.
Eventually when this project reaches a good state I’ll see if I can get it on the extension stores, but for now you have to manually load it.