If you’ve written PowerShell script’s to create virtual machines, attach the ISO file to the virtual machine, boot up the VM, install windows, and then more PowerShell scripts to join the domain or use PowerShell Desired State Configuration to configure the whole virtual machine; the last thing you want is to do is get hung up during the Windows Installation by having to click through it.
Keep it automated.
WinISO can be used to update ISO files. This is not from Microsoft and is 3rd party. The free version will work just fine for what you need to do which is add an autounattend.xml file to the root of the ISO image.
WinISO can be found at http://www.winiso.com/products/winiso-free.html