How to process a custom form field

Hi,
I’m evaluating s2member plugin because I’ve this need:

I want to have a registration form with a custom field (named “coupon code”) in which user input a code they got buying our book

If the user input the right code he can register for free otherwise – if he hasn’t the code – he can leave the field blank and have to pay

Is it possibile to do it? If not have I another way to have this accomplished?

thanks in advance
Paolo