if you need more than the 12 fields available under wppizza->order form settings try the following filters in your functions.php (As of 2.9.4.6+)

/******************************************************************
'sort' => sets sort order
'key' => should be unique (otherwise there isn't really any point)
'lbl' => label of the field
'type' => text, textarea, email or select 
'enabled' => should really always be true (otherwise there isn't really any point doing this) 
'required' => makes field required to be filled in
'required_on_pickup' => makes field required to be filled in even when self pickup eabled
'prefill' => prefills field (if available)
'onregister'=> adds field to registration page
'add_to_subject_line'=> will add output to subject lines of email
'value'=> array() if type =email,text or textarea, array('val1','val2',....) if type=select
'placeholder'=> set a placeholder for formfield
*****************************************************************/
add_filter( 'wppizza_filter_formfields_register', 'my_order_fields');//registration page. onregister must also be true
add_filter( 'wppizza_filter_formfields_profile', 'my_order_fields');//profile page.
add_filter( 'wppizza_filter_formfields_order', 'my_order_fields');//order form.
function my_order_fields($formfields) {
/**text field (set type=email if email field, type=textarea if textarea)**/
$formfields[]=array('sort' => 0,'key' => 'billingname','lbl' => 'Billing Name :','type' => 'text','enabled' => 1,'required' => false,'required_on_pickup' =>false, 'prefill' => 1, 'onregister' => 1, 'add_to_subject_line'=>false, 'value' => array(),'placeholder'=>false);
/**select field**/
$formfields[]=array('sort' => 0,'key' => 'billingcountry','lbl' => 'Billing Country:','type' => 'select','enabled' => 1,'required' => false,'required_on_pickup' =>false, 'prefill' => 1, 'onregister' => 1, 'add_to_subject_line'=>false, 'value' => array('United Kingdom','United States'), 'placeholder'=>false);
return $formfields;	
}