You don't add users to a second domain, you leave them where they are. However you should tell your section access table that some Users are defined in another domain by specifying DomainY\User2 and DomainZ\User3 as NTNAME values. Don't forget to assing CALs to DomainY\User2 and DomainZ\User3. You can only do this if you define DSCs for the other domains, or QlikView won't find the definition of these users.
Contact your IT administrator. That's a Windows AD configuration job and I guess you won't be allowed to change such things by yourself.