Sorry, that's not possible. Replacing the stereo with an aftermarket HeadUnit, will render those controls useless. (Same for the G1). There is one possible way, which I have not tried.
You can buy an aftermarket Head Unit with IR Remote Control. Now, because it has IR Remote Control it has to have an IR Receiver on the deck. There is a company called "Pacific Accessory Corporation". They offer lots of neat little things to help with aftermarket systems.
Now, from what I gather. This little device hooks up to the Volume Controls on the steering wheel. A wire intercepts the signals... and then turns it into an IR signal. This IR signal (programmed from your units IR remote control), will send it throughout the car.
So every time you tap your volume up/down.. a little device would send an IR signal to volume up / down.. your IR capable deck would pick it up and act as if were receiving commands from the remote control and move the volume up / down.
I DON'T THINK IT'S POSSIBLE SINCE THE WIRE THAT PERTAINS TO THE RADIO CONTROLS ISN'T COMPATIBLE WITH AFTER MARKET RADIOS, NOT EVEN PIONEERS HAVE THAT CONNECTOR, BUT I'VE HEARD THERE ARE KITS AVAILABLE TO STILL HAVE RADIO CONTROL FUNCTIONS FROM THE STEERING WHEEL FOR AFTERMARKET RADIOS. HAVEN'T SEEN THEM MYSELF THOUGH