I think I tried to answer your original question. You were correct in that SBS cannot be used as part of a larger network, nor can trust relationships be set up with other domains.
By your wording here, it appears you don't have any domains setup at present (you can't have a domain without a domain controller for a Win2K domain or a PDC for a WinNT domain). You have a peer-to-peer network where each computer/server that hosts a shared resource (printer, folder, etc.) is responsible for authorizing access to that shared resource. (i.e. there is no centralized user account management).
The simplest way to go about this would be to create user accounts in SBS that use the same user ID and passwords as are presently being used to access shared resources. Windows NT and 2K can then make use of what is known as "pass-thru authentication" to allow users in the SBS domain to access shared resources in your present peer-to-peer network.
The one possible problem you will have will be that users who have there workstations joined to the SBS domain will not be able to browse the peer-to-peer network for shared resources. You can get around this by mapping the shared resource using a UNC path (usually using the format <\\servername\sharename>). Once user computers are joined to the SBS domain, the SBS DC will not be able to resolve any servernames that are not part of the domain. However, you can also map a shared resource using the host computer/server's IP address (i.e.<\\hostIPaddress\sharename>).
Whether you install an NT4 PDC (to create an NT4 domain) or install AD (to create a Win2K ACtive Directory domain), will not change the need to follow the above procedures to all SBS domain members to access any shared resources outide of the SBS domain.
Let me know if you have more questions.
