Prestashop modifications

I have prestashop project and I need 2 modifications: A) add a new field 'carrier account number' which will be displayed in the 'Shipping' phase in the checkout process under carrier name, so that when customer fill this field then shipping cost will not be charged. And ofcourse this field should be appear in backoffice in order details, and admin may edit it B) Add a new simple payment gateway module (details later)