Payment plugin customization

-- HikaShop version -- : 2.3.x
-- Joomla version -- : 3.3
-- PHP version -- : 5.6
-- Browser(s) name and version -- : Firefox 30

I'm creating a custom payment plugin and I would like an extra form to get some information from the customer.
I want to add it in the list of payment choices that are presented to the user when he is in the checkout process, inside the area printed by the custom plugin.
How could I do this?

Thanks