You'll have to use the MS tool for it. That is the only way to migrate. I have heard good and bad stories about it.
The tool is called the Exchange 2000 Migration Wizard and comes with the Exchange 2000 Service Pack CDs. See the following Microsoft articles to get a grip on the Wizard:
- Introduction to the Enhanced Migration Wizard
- Using the Exchange Migration Wizard to migrate mailboxes from an Exchange organization
This was first published in April 2003