* User1 - Yes
An Enterprise admin can promote a workgroup server to a domain controller within an existing Active Directory subdomain. This process involves installing the Active Directory Domain Services (AD DS) role on the server and then using the AD DS Configuration Wizard to add it as a domain controller for the specific subdomain.
* User2 - Yes
User2 is domain admin in the subdomain.
Reference:
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/deploy/ad-ds-installation-and- removal-wizard-page-descriptions