While we didn't ask about HUD, we had the same issue w/ the power liftgate (only missing option we wanted). The dealer said it could NOT be added after the fact. I'm guessing the HUD couldn't either, except possibly as a true aftermarket mod, as Adam mentioned.
I had to have the HUD unit in my Grand Prix replaced under warranty. It sits in a hole cut in the top of the dash in front of the driver, and the controls were mounted in a knock-out in the dash. Doesn't look like a big problem to install the physical unit, but the actual electrical interface might be a problem. Check with your dealers service dept. Good luck! I really like mine. After market might also be an option.