The import tool (in the Unity program group) allows you to import new users from a CSV file and it'll create NT and Exchange accounts for them on the fly. If you're users are already represented in Exchange the tool lets you import them as suscribers from Exchange.
If the later is the case, you may want to prepopulate the Business Phone field which is what the import tool looks at for the user's Extension number by default. Here's a post on how to use Microsoft's header import tool for doing that in bulk from a CSV file if you're interested:
