Comentário oficial
1) You won't be maintaining any kind of consistency between the imported playlist and the iTunes playlist. Use the Sync Manager instead (click down at the bottom left of rekordbox), and select the iTunes playlists you want to synchronize over. Next time there are changes within iTunes you want reflected in rekordbox, run the Sync Manager again and click the big arrow to re-sync.
2) Go to iTunes > Preferences > Advanced and check the "Share iTunes Library XML with other applications" box. Click OK and close iTunes.