Spotify allows you to import tracks from your computer and add them to playlists as if it was on Spotify. Obviously you can only access them on the computer you added them from though.
Not sure about the mobile app, I assume it will skip the missing tracks in a playlist as normal but it might be possible that it picks them up if they're present on the device somewhere.
Not sure about the mobile app, I assume it will skip the missing tracks in a playlist as normal but it might be possible that it picks them up if they're present on the device somewhere.