Welcome to the boards.
If you are not familiar with PHP, we cannot teach you here. PHP is a fully featured programming language. You MUST know at least the basics in order to start tinkering with other people's code. I suggest contacting the vendor who sold you the shoppingcart program and asking them how you can add that feature.
Usually the shopping carts that are already pre-programmed for you - allow you to simply go to a control panel and turn these features on and off.
If you go in and simply add a field - you could open up a security hole that you would rather not have. You could also break their code and the whole program can cease to function.
Sorry we can't be of more assistance to you. If you do not have the time to learn PHP or the program itself, I can also suggest geting a freelance PHP Programmer to do it for you. There are sites out there that specifically match you up to programmers - however we cannot promote them here.