Using Drupal with separate membership info

We have a large database of members/customers who are currently assigned usernames and passwords based on the membership number that is assigned when they subscribe to our magazine. Our current web solution integrates directly with our database, albeit badly.

How easy is it likely to be to perform an automated account creation when new members call our national office? Ultimately, we want to restrict access only to paid members and not allow non-members access to other content.

Thoguhts? Suggestions?

Thanks loads!