My understanding is that all you would need is your reglular Yahoo accounts, no Plus account needed. mobile.yahoo.com/blackberry shows that it will push email from your Yahoo account to the handheld. (One-way synchronization between your BlackBerry device and your web based Yahoo! Mail account. When you make changes on your BlackBerry device, it's reflected in your web account. (And two-way sync is coming soon.))
When you compose an email, you can set which account it is being sent from by toggling the Send Using field at the very top of your message. Also, when replying to an email...it uses the address the original email was sent to.
I guess I'm just not understanding why you would want to set it up the way cdysthe proposes.