When playing music in Betaplayer, is there any way to enqueue music, ie. to add it to the end of the current playlist, through Resco explorer or something like that? I find the way you can add files (or directories, if you want all files) to a WinAMP playlist to be extremely useful.

No it isn't possible nor is it desirable to implement this feature in such a way that it requires an additional program. You can already add multiple files/directories from within Betaplayer so the need for such a feature is debatable.