Piano Marvel allows you to add your own pieces to the library, I did it with one I found online. What they recommend is using Musescore to convert the MIDI to a score, fixing it up in Musescore and exporting as MusicXML. Musescore did a creditable but not perfect job of the conversion, but fixing it up was easy. Then you import both files into Piano Marvel. I'm actually not sure why they need the MIDI at all once you have the MusicXML, given that Musescore can play the music from the score directly.