Ah, you mean have the file opener as a playlist. Unfortunately, I don't believe that's possible--the music window's only method of opening these mp3's is coded as the dropdown window. Unless you mean the actual file opener window, the one that appears when you click "add", in which case, I agree, that would look very cool along with the winamp mod, even if it wouldn't stay open all the time.
Only suggestion is that if someone makes it, they use the actual matching default winamp playlist editor, instead of that greyscale one you've got in the screenshot.
EDIT: Nevermind, it looks like you've actually gone and made the window yourself, as it would look in EQ, but with the winamp style, which is very nice, it's a good edit ^.^ Still, it should be colourized, I think, if you want it to match