Hi all,
I try to create a module which will add 'NIP' (it`s polish tax number) field to address form.
I try to use hook_form_alter, and hook_checkout_pane, I will also try to use module from this thread to store all address information in user profile. I will create separate table with two rows 'uid' and 'nip', to store NIP information.

I need some advise, which form and how I should alter to get something like this:
*First name:
*Last name:
Firm:
NIP:
*Street address:
*City:
*Postal code:
*Phone number:

You have to fill 'NIP' field only if you filled 'Firm' field.

Best regards (and sorry form my English).