Windows2000 works a little differently than NT4 - every W2K server is a "member server" until Active Directory is installed and enabled.
A W2K server is joined to the NT4 domain just like an NT4 member server would be, although the menus are in alittle different place. From Server Setup, click Networking and then Network Identity. From that point, it looks very much like the NT4 routine, choose a server name, choose the domain, it will ask you for a userid and password that has the necessary rights to add a server to the domain (usually administrator), it will rummage around a little and then give you the Welcome to ... message.
WHen you upgrade the PDC later to Windows2000 and then do the Active Directory thing, the machine account will already exist and will be properly migrated.
paul