Setup Magento CPF field

CPF is an unique ID number in Brazil, which means one CPF per person. So ppl don't like to share their CPF's so easily in websites. With this said, we want the CPF field to be optional in our website. Maybe the person just wanna checkout the page, or just sign our newsletter for future buying. In this case it don't needs to fill up the CPF field. But in the future, they will need to fill the CPF field to make checkout. We are using an customised "one step checkout" extension, thats is NOT the traditional/original "One Step Checkout", and its called O2TI. So please make a research about this plugin before applying to this project So we just want the person to fill up the CPF on the checkout page, and once filled, that field must not appear on the checkout page again. If the person already filled the CPF when registering, it must not show up on the checkout page. The person must NOT go out from the checkout page in any case. If needs to fill anything, after submitting he must stay on the checkout page. Resume: CPF field must be optional. Person needs to fill this field if wants to buy but don't need to filll if it only wants to register. Once this field filled, our wbesite must not ask to fill it up again. Person MUST STAY on the checkout page. A CPF must be owned by only one person We are using Magento 1.6.2 As development tool we are using Springloops and SVN checkout. FTP deploys won't be allowed. Only SVN commits (every commit goes directly to server) **We will only work with developers that agrees with to work with these tools**