Automatic Mailbox Creation

I have over 200 users on my NT 4 server and anticipating 600 more. What I need to do is create thei mailboxes in Exchange 5.5 without doing manually one after the other. The problem however is that the users register on a daily basis. I import theaccounts from a CSV file but I am not able to do that in exchange. Is it possible and if so how. Thanks in advance