I don't know about yahoo, but I have set up my gmail account to forward messages to my BWC address. When gmail forwards the message, it retains the FROM: address of the original sender, so replying works just fine. Also, on the blackberry.net website, it was easy enough to set my FROM address on the handhled to be the gmail address, so forwarding and composing BWC messages also appear to be from gmail.
Repeated the same with my hosted BES account and work email.
So, I think I'm doing exactly what you want to do.
If you want a gmail account, I have referrals, just email or PM me.
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