I think you may be trying to do something that your phone does already....
I have 3G on during the day at work and turn on Wi-Fi when I get home. The Wi-Fi icon appears, and the 3G icon vanishes. While the Wi-Fi is connected, the phone transmits exactly zero data over 3G (confirmed by checking my 3G Watchdog app). When I move out of Wi-Fi range the 3G starts working again.