This game is actually pretty good for modding because of the github page so you could actually send ninj your haxe files over if you want your mod in the game i'm guessing.
Viewing post in Friday Night Funkin' (Ludum Dare Prototype) comments
it wouldn't be too hard to make urself custom charts, but right now it would be a but clunky. I need to rework the charting system to probably be in-game or use some other charting program like Moonscraper (used for Clone Hero). Although I imagine at some point I'd want more custom scripting type shit and I'd have to implement it or do some hack-y shit myself. We'll see!