If NT registry hive is not mounted, you can mount (load) it yourself.
- Open regedit.exe
- make sure HKEY_LOCAL_MACHINE is selected on the left panel
- click File -> Load Hive in the main menu
- open ntuser.dat file in common dialog
- specify some temporary name, like USER1
- make changes
- make sure HKEY_LOCAL_MACHINE\USER1 is selected on the left panel and then unload hive from main menu
Registry hives are also loadable and editable from the command line:
REG LOAD /?
REG UNLOAD /?
So you may call it from your Powershell or batch script.
But all local user hives are usually automounted in HKEY_USERS. So you need to manually mount only foreign registry hives.
P.S. I prefer the term "mounting" instead of "loading" because loaded registry file not just loaded to RAM, it's locked and being actively changed on disk, so it can't be copied away until dismounted/unloaded.