Custom Payment Gateway with extra text fields

[shellsensory]

I'd like to be able to create a custom payment gateway that would allow extra text fields for the customer to complete. For instance, we have a payment option where customers can use healthcare funding as payment. They would enter their customer name, number and provider email address, and we would send the invoice to the appropriate provider for payment. 

Or if a school is ordering with a purchase order, a field for them to enter the purchase order number and accounts department email address. An invoice would then be generated with the purchase number included, and the invoice emailed to the relevant email address. 

I want the extra fields to display only when that particular payment method is selected, they are not relevant to other payment methods.

Is there a way to do this?