1.90 , how do i add social security-field at the checkout

Hey,

I need to add a social-security-field when the user wants to make a checkout, but i don't want to save this ssn to the user-attributes.
I can't figure out how it works..., how do i send a value to the PaymentProcessor and from where?
I guess i add a textbox in the template for the paymentmodule?
I have created a property in the PaymentProcessor-class for this.

Very grateful for help!