There's nothing in Bitmelo specifically that can do it, however you might be able to get it working using the browser webaudio api. The files would have to be hosted somewhere else, and I haven't tested it out. It might not work though because the game is running in a sandboxed iframe.
The editor doesn't support more than 1 palette, however you can swap the palette in game by using the bitmelo.screen.setPalette function. The palette has a max of 255 colors.
PNG exporting should be coming in the next update. PNG importing will be either in the next, or update after that as it's a bit more complicated.
A music tracker is the next big feature I want to add, but it may take a while as it's a pretty big undertaking.
Yea the github for the editor is where I'd prefer bug reports, but I should be able to see it either place.
