Altough this won't work so great for constant syncing, this one-time trick worked for me: I uninstalled the newer version of Spotify, downloaded an older one that imported from iTunes (in my case version 1-0-72-117 from this website: https://spotify.en.uptodown.com/windows/old), successfully imported all playlists from iTunes, and then updated Spotify to the most current version. Done in a matter of a minute or two and no need for 3rd party apps!
Thanks for sharing this! This is perfect for me because I listen to Spotify on my work computer but keep all my music in iTunes on my home computer. As I work through all the music I've downloaded from NoiseTrade, I add stuff I like to a "Work Mix" playlist, which I then run through http://smarterplaylists.playlistmachinery.com to do some artist separation.