You can't override the system. I looked and asked about it. My car had 20" rims on it before I even purchased it (the dealer had them installed) and they didn't do anything to make the TPMS system work. But I know there are straps and sensors out there. You should be able to use the sensors from the stock wheels and buy a different size strap. I don't know where the best place to get them is, but I know www.tirerack.com
has them, although they look pricey on there. Your best bet would be to go under the tire section in here, there are a lot of questions on there. Colin from the above website has a wealth of information too, and he won't sit there and try to sell you their parts. Oooorrrrr, you can just suck it up like I am and hit the "reset" button every time you start the car to clear the TPMS malfunction warning.