Hello,
First of all I apologize for my wrong answer. Indeed, on a domain controller local accounts are disabled. Also, when running an installation package (which contain an user added in our "Users and Groups" page) on a domain controller, then the related user will be added as a domain user (tested on a Windows Server 2003 promoted as a domain controller).
So, in order to achieve what you want you can use the USERDOMAIN environment variable. Here's attached a sample which creates a "TestUser" user account and set (on "Custom Action" page) the MYPROP property to USERDOMAIN environment variable value.
Let us know if you have any questions.
All the best,
Daniel