Unfortunately I don't know anything about SBS 4.5, but I assume it works just like NT4.0 server. Can you tell me a little more about it to clear up my confusion?

If my assumption is correct about SBS, then here goes:
Did you add the 2k server to the existing Domain, or are they two different Domains? If it is two different ones, you will need to establish trusts going both ways, or add the 2k server to the existing domain. If the 2k box is not a Domain controller, you will need to add it to the domain, and not just a member of the workgroup. Then you should be able to map to shares with a domain user, provided the shares permit those users.

If this is all a workgroup environment, then forget it, I misunderstood...and none of this will help; unless you want to create a domain!