Dude, just download wc3iconlayer from the tools section.
If you have photoshop, you can do it yourself in just 2 minutes or so.
It works like this:
1. Extract the icon using wc3viewer and convert it to TGA (if you havent already got it)
2. open the tga in photoshop, then hit CTRL+A, CTRL+C
3. open wc3iconlayer (which is a psd document) in photoshop, hit CTRL+V, then just put the passive layer on top. There you go!
Finally, save it as TGA, convert to BLP using wc3wiewer (or anyy other similar program that has the same effect), then it's done.
This can be done with basically any icon you want.