Migrate from Nt4 to sbs2003

Hi, I have following problem. We have an old NT4 server in domain NTdomain and SBS2003 in SBSDomain. I need to migrate users and groups, then upgrade NT4 to server2000.The biggest problem is how to change security for shared dirs for example from NTDomain/User to SBSDomain/User on server2000.Is it possible to add server2000 to domain SBS2003 and make it DC using DCpromo?

used following:
1)migrated users/groups from nt to sbs2k3
2)resinstalled nt to server2000
3) and now need to replace sharing nt/user to sbs/user