advanced registration module for Joomla 1.7

I need a plugin (no hack) to add fonctionality to the the registration form in Jommla 1.7 : Let's imagine 3 user's group : FORD, FERRARI, PORSCHE ... Future users are supposed to had received by mail from their direction a secret code to register on our website . We need to add the "secret code" field in the registration form and use this code to assign new user to a specific group. User who register using secret code "ITALY" will be assigned to Ferrari's group, User who register using "USA" will be assigned to Ford's group and user who register using "GERMAN" will be assigned to Porsche's group.