I think most of us import our iTunes playlists or whatever music player-playlist that we have, and then we build on those playlists through Spotify, however since there's a lot of music that is not on Spotify yet, if you want to have a song you have to buy it through iTunes and then add it to your spotify. Long process, yes.
Anyways, my suggestion is to have a "Sync" option added to the menu where you can sync playlists from iTunes that have already been added, that way you can add new music that you would have bought, into Spotify, without changing or deleting tracks that you have added from within Spotify.
Please bear in mind that the Community is not an official Spotify support service. It's a place where we all help each other, whether we work for Spotify or not. So please use your discretion when using the forum.