Well, not completely at least. This is what is the conclusion after trying for some days now to add extra fields in Ubercart's cart checkout page. As I found out in Stella's blog, I have to use
hook_checkout_pane()to create a new pane in checkout page. It was a bit complicated at first glance and I didn't have enough time to work it out this way. More specifically, I wanted the user to fill-in a form, saving extra information to the system, before being able to really checkout. What I did to solve the problem I was facing, was to use the usual hook_form_alter() function to create a new [#validate] callback for the form. In this validate function, I checked whether the function was filled with the necessary fields and if not, a form_set_error('submit','message_shown_to_the_user_here') is called.