You can use a resource kit tool call ADDUSERS.exe to dump the user information from NT 4.0 and move it to Win2k. However, the SIDs will change presenting some file/folder access issues on your memory servers. You will also experience permissions issues with User Profiles on users' workstations.
You can move a Win2k member server from NT 4.0 Domain to a Win2k domain. Just take it out of the NT 4.0 Domain and put it in a workgroup. Then, join the Win2k domain. If you have complex file permission settings, you will want to have a plan and some time to migrate the user account access permissions (adjusting the ACLs - check out CACLs from the resource kit or FastLane Technologies for scripted solutions).