Music application that integrates with iTunes and shows the waveform of the song

You can try what this fellow suggested as an answer because I think you can easily import iTunes tracks to the Spotify application.

That's the only way I can think of.

The columns and metadata that iTunes supports is hard coded unfortunately. There's no way to add a custom field or column to the browser.

You might be able to get a similar effect with some combination of smart playlists and/or some external scripting, but there's nothing built in that will do what you need.