Try these ...
http://forum.x-cart.com/viewtopic.php?t=1084&highlight=registration
Basically the best way to do it is to use a field that you wont be using in the customer registration form. Usualy website / ssn / company etc.
you would need to use the field in the database to store the result of the drop down menu that you have added to you form in register.tpl and then change all your mail templates to show the results.
If you want this done for you as a mod then get in touch