Form at payment method ? Please help

Hello. My name is Valentin and I'm new to Opencart and PHP also. I use Opencart 1.5.6.4 and I am completely satisfied with the software until now. Now, I need to insert a form like a payment method, so, the buyer can insert additional needed data for payments (reference number, and so on). I would like to insert it as a payment method, but I can't find any solution to modify the Cash on Delivery or Free Shipping payment method . Any solution is well received, I hope to hear from you guys soon. Thank you very much and PLEASE help