Credit Card Payment Plugin - Adding billing zip

My client is looking to use his 'square app' (credit card processor for his smart phone) so people don't have to see the paypal screen. The credit card payment plugin is ideal, but I'm wondering if there is a way to add a custom field to the drop down form on the checkout page that could be used for the billing zip code, as this information is necessary for him to process the card..

thanks in advance.

Got that! Now, I've received an order, I understand that there are two separate numbers (first half) + (second half). I received the email with the end of the credit card number, but cannot see the beginning.. When I looked at the administrator> hikashop> Sales> current sale, the partial number was there, but now it isn't.. hope that wasn't too confusing.. just wondering how to access the beginning of the number... thanks.

note: my orderhistory plugin was activated before any orders were ever created.