I would do it, but I don't have any programs at this computer. But hey, It's not that hard:
1. Find an image of a pokeball
2. In wc3 extractor 2, You can find icons. Just take one, and export it it. Open that icon in any editing program, and simply paste in your pokeball and make it fit inside the ''frame'' of the icon. Then, save the thing in TGA, and convert it to blp; Which you also can do in wc3 extractor 2.
Not shure which path the new icon would go under but...