The standard Storm theme doesn't allow you to do that. You can download themes that have this and look A LOT like the standard theme, but it isn't possible with the stock theme.
The only way you can get an icon up there is to download a theme or make your own. Other than that, it is impossible.
There are TONS of free themes in the themes section that have a slot there just for the weather apps. Or if you download the free theme builder from RIM, you can just make your own. It's really easy to make themes.