I'm not sure if you're trying to do something more complicated than it sounds like....but if all you're trying to do is take songs from your external drive and put them on the internal one, you just grab the files and drag them from one folder to the other. Then, add them to the music player of your choice.
However, if you're trying to grab JUST the songs on that play list, that won't exactly work the way you're trying to do it. As said above, the playlist is just a bookmark, or a table of contents. It tells the music player to grab specific files from a specific folder and play them in the order they're listed.