There is a request thread for it:
FEATURE: Using own music!
But like Line said, it’s unlikely that we will get to do that, I suppose the closet thing would be perhaps adding a readerMessage that lets the readers know that you have a certain song in mind for whatever scene and including the song name and artist. There’s also the 5 sentence limit per chapter that you could use for characters that you want to sing the actual lyrics of the song, but there’s nothing else that we can work with to incorporate more of the songs we’d like outside of episode’s own music. 