If a song is cached for offline playback, it is still not possible to use the download function to copy it to the download folder. The media provider must be online for that, and I guess the file is being downloaded from the source again then.
Is it possible to enable that?
Is this really necessary? This creates issues for most cases when things are only half cached for example. I’d say that this is a very rare case.
I thought of being out of your network, e.g. at a friend’s, and wanting to share the music files. Usually an easy thing for local music players. With the offline cache, the file is there but cannot be located through the app currently.
I am not sure if this is really an important use case, though.
Adding the option for 1 song on the 3 dots is not an issue, but adding that to the rest like genre that can only be half cached is problematic to when to show the menu and how to warn the user only a part will be copied.
If I were to use that, I would probably not use it higher than the album level. Artist might still make sense, a whole genre not so much. Having it on a song level only would be cumbersome though if you wanted to download an album.
I would say enable that on song and album level (if at least one song is available offline), and on the album level, if not all songs are cached, show a dialog with a notice, either generic (“Not all songs are available offline.”) or more specific (“Only 3 out of 10 songs are available offline.”). Then the user can decide if he wants to continue or not.
Well I won’t go adding dialogs and everything for a 1 in a million use 
So for the moment I’ll close this sorry.