Register users based on a list

Our company is in the odd situation of wanting to allow our employees…and only employees…to register on our site to see company documents. We have a frequently-updated list of employee numbers that we would like to use as the check against whether new registrations are employees. Ideally…we would like the standard registration fields (ie, username and email address) plus a 3rd blank for employee number. Cimy extra fields plugin does a great job of adding the field and recording it…but we need to find a way to reference the employee numbers list and/or import it into the database each month. Any suggestions?