Welcome to Android Central! It's not exactly clear to me what you're asking. Are you trying to combine music you've bought and downloaded from Google Play Music with other music that you downloaded from elsewhere? You generally can't play downloaded music tracks from Google Play Music on other music apps, because those tracks are in a hidden folder, with a coded filename. In order to save a Google Play Music track on your phone that a 3rd party music app could play, you'd have to download the track on your computer from your Google Music library website, then transfer that MP3 file over to your phone.