AFAIK windows doesn't allow more than one domain, so it's not possible
Windows does allow for that, but not out of the box. It involves Active Directory and inter domain trusts (http://technet.microsoft.com/en-us/library/cc786873%28WS.10%29.aspx) , which is not possible with SME Server as it is a NT based network neighbourhood (as it is using samba).
Is there a change (NT 4 domain) in future, maybe with >= 8.0 ... ?