For minim, you can use a relatively simple shim such as https://github.com/Pomax/Pjs-2D-Game-Engine/blob/master/minim.js - include it as a normal JS script on your page before loading any sketches, and your sketch won't complain that minim is an unknown thing.
Because Processing.js compiles your sketch to native JavaScript, global objects like Minim can simply be emulated in JavasScript as well: as long as the functions you're calling for Minim are defined in your JS "shim", things should work just fine.
On a more general note, because Processing libraries are precompiled Java VM byte code, Processing.js cannot load them, so any import statement is ignored.