How to add form in checkout process?

[Ryouji]

Hi, 

 

I have a litle problem with my checkout process.  I want to put a form if the quantity of products in my cart are 2 or more, I made this logic and include the form ok in the shopping.cart.tpl in the default theme of prestashop but ... now how I save the data of my form in ps_order in my db? for later see that in order information in backend.

 

Thanks.