I have no idea how to do that. But if you have an iphone and want to know another method to download music to your phone you can try this:
What I do is download it directly from Youtube using Youtube ++ in cydia
With this tool you can do more than just that but one of its features is that it allows you to download YT videos as only sound (MP3) or both sound and video (mp4) with different resolutions, once you donwload any song you can export it from your library (inside YT app) to your music (the deafault music app in your iphone) allowing to also edit name´s song, artists name, etc. I like this more because this way you dont need internet connection (if you use spotify and you are not premium) and you can have a wider variety and not limited to Spotify
Hope this helps!