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.