To add a new character, just copypaste an existing png of a character's spritesheet and draw your new sprites on another layer, followed by copypasting the character's xml and json
To make a new song, use your music composition skills to make a song with an audio file called a chromatic¹. Then save the song's chromatic melodies named "[song]vocals" as ogg, then the rest as "[song]inst" as ogg as well
To chart a song, copy a song's json, then rename it to the song you made, clear the chart and chart it like normal