I am not an expert on travel visas, but my understanding is that anyone entering the USA without a visa on the Visa Waiver Program can visit for only 90 days. If they leave the country to visit Canada or Mexico or one of the Islands in the Carribean, they can reenter the USA, but they must still depart by the original expiration date of 90 days.
You cannot extend your stay by going to Canada or Mexico for a short visit and return to the USA. Your original expiration date on your I-94 form will always stay the same.
If you come to the USA with a visa that allows you to stay 90 days, you can contact the US Immigration and Naturalization Service to request an extension. They can permit you to stay longer. If you come to the USA on the Visa Waiver Program without a visa, I believe you cannot get an extension.
I think you should contact the US Immigration and Naturalization Service to ask them this question.