Add more fields Please help

I am using MarketPress plugin + GridMarket theme. There is information I need to get from my customers before they checkout. Is there a way I can add a custom field on the products page. Example: on the home page you sell all products. You click on the product you want and it give you information about the product. I would like to add a custom field on that page and when you press the buy now button have it to save in the shipping info. Please give me the code and what lines to add it on. Thank you in advance.