If you do not want all your OnSong backing tracks scattered about in your music folder listing with your normal music tracks, you might want to prefix the titles with letter 'z', a number or other text character, so they all appear together in one place; or perhaps use an album name called OnSong.

But it might be possible to add audio directly into OnSong's Documents folder or import tracks from Dropbox into a song (iReal Pro can save audio directly to Dropbox using 'Open in' if you have the Dropbox app installed) but it was not immediately obvious in OnSong how you access either of them. You might like to contact OnSong support to check.