How to Add a Song to an iPhone Without Syncing
Syncing your iPhones to iTunes on your computer allows you to download all of the files in iTunes, but it can also accidentally change your settings and music without your knowledge. If you want to fill your iPhone up with music without going through the process of finding your cord, plugging in your phone and syncing with iTunes, you can use the on-board version of iTunes that comes pre-installed on your phone for quick access to music.
Touch the iTunes Store app icon on your iPhone home screen. Sign in using your Apple ID.
Touch the music icon on the bottom of the screen. This will allow you to browse through music available for purchase through the iTunes store.
Touch the price tag beside a particular song to confirm you'd like to purchase the track. You may need to sign in with your Apple ID once more to initiate the download.
Close iTunes and navigate to the iPod icon. Touch to initiate and to see your new music in your playlist. Touch the "Playlists" icon at the bottom of the screen and then selected "Recently Added." As long as your song has finished downloading, you should see your new music available for play.
Transfer your new music to iTunes on your computer the next time you sync by selecting your device in iTunes, clicking "Music" and ensuring that "Sync Purchased Tracks" is selected. The song will then appear in your iTunes library.