Hi,
I am good experience in AD and Powershell, can help you in joining the workgroup computers to the domain using the powershell script.
Regarding the user account, have one question, are the current users are created in AD or locally in the server?
If the user created locally in the server, than we can use script to create a new account "xxx_user1" in the domain, if the existing user is already created in the domain , can we create new user name in the new domain or do you want to migrate?
Anything in the AD side I can help, please let me know.
Thanks and Regards,
Saravanan