Sorry, on BIS2 I don't think there is native support for what you want.
You have set your blackberry.net email address to add a reply-to with a different address and that is what it will do. It probably takes place on the server side.
I may have a solution for you soon by using Gmail as a shell account. You could get all your main email instantly sent/received and keep your current non-gmail address. Stay tuned to this thread Gmail BIS2 Setup
johnfromphilly - My Gmail setup expects you to specify the reply-to address in the BIS settings for your bb.net address. I only mention since you posted you were not on BIS2, and I do not know what options are available on the older release, or how it will react.