in your themes functions.php

/**only a-z A_Z 0-9 _ - everything else gets stripped, spaces get replaced with underscores !*/
/**statusses already in use in the db can not be unset nor can the "NEW" option as it's the default**/
/**BACK UP YOUR WPPIZZA_ORDERS TABLE FIRST . YOU HAVE BEEN WARNED ***/
add_filter('wppizza_filter_order_status','change_my_order_status');
function change_my_order_status($args){
$args[]='custom status field 1';
$args[]='custom status field 2';
return $args;
}

BACKUP YOUR DB FIRST BEFORE YOU TRY/DO THIS.
it has been tested, but better save than sorry i would say…..