Wirelessly posted (8700g: BlackBerry8700/4.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100)

I don't see a way to do it.

I know this doesn't answer your question, but what about setting your yahoo accounts up in BIS? That's the best of the alternatives, in my opinion.

Another alternative is to use the yahoo mobile site, but I expect you already know that, too.

Good luck.