A lot of software can import MIDI and export MusicXML. But if you are writing MIDI to NWC, you know that MIDI does not contain everything needed to do a score. You can download <a href="http://musescore.org">MuseScore</a> to open a midi file, fix the score and export to MusicXML. MuseScore is free, 100%.
You can migrate your scores with http://nwc2musicxml.appspot.com/ to MusicXML. It's not perfect but might save you some times. If you miss some features let me know... On Mac, you can try MuseScore: http://musescore.org It's free and open source. The Mac version is still young but is improving every day.
Hi, I decided to work on nwctxt to MusicXML convertor to open the large library of existing nwc scores to other programs. My very first result is at : http://nwc2musicxml.appspot.com/
You just input or upload a nwctxt file and get a MusicXML file in return (if it works) You can open and edit the resulting file with MuseScore http://musescore.org. MuseScore is free and open source.