Modify Checkout Step

-- url of the page with the problem -- : regalodifiorisumisura.it/shop-categories/checkout.html
-- HikaShop version -- : HikaShop Starter 2.2.3
-- Joomla version -- : 2.5.17
-- PHP version -- : 5.3.25
-- Browser(s) name and version -- : Firefox 15.0.1
-- Error-message(debug-mod must be tuned on) -- : none

I would like to add an addictional field in the order, in a checkout step just before shipment address ( see url of the page with the problem). I would reach this behaviour: regalodifiorisumisura.it/regalodifiori/f...to/personalizza.html
In the textfield of the step-page there will be inserted a text that I need to save in the order.
Can you suggest a clean way to reach this result?
Now I've added form.php tmpl in checkout tmpl folder and I've copied function form from address/view.html.php to checkout/view.html.php.
I call it adding "form" layout to step() function on checkout/view.html.php, only in step0.
I could modify a view that I don't need to use in checkout process.
Thanks.