Question : How do I configure an alias email account on my iPhone?

I have an email address that forwards only to another email. So, email B forwards to email A.
Email A is configured on the iphone. I want email B to show in the from address when replying to email sent to B and send from A for email sent to A.

Since B forwards through A, I use the A pop and smtp settings. But iphone won't accept it, Says A is already configured and forces me to either edit the info or cancel the attempt.

I use a 3G iphone

Suggestions?

Answer : How do I configure an alias email account on my iPhone?

I don't think the way I describe that I want to do this is possible at this time on the iPhone. At least, no one has been able to show me one.