Nightingale allows you to import your library from iTunes. Not uspported are Smart Playlists. Also keep in mind, that Nightingale can't play tracks with DRM.
Go to Tools→Preferences and select “Media Importer” in the Preferences Dialog. Click on the “Browse…” button and go into your “iTunes” folder. You should find a file called “iTunes Music Library.xml” in it. Select it and click “Import Library”. Nightingale now imports your library from iTunes.