Assuming you want to modify the HKLM hive, do it like this:
REG LOAD HKLM\Software1 C:\Windows\System32\Config\software
Then modify everything in HKLM\Software1 that you would normally modify in HKLM\Software
If you want to try this from the GUI, just click on HKLM and then click File --> Load Hive, and browse to the file name C:\Windows\System32\Config\software ("software" is a file with no file extension, which is the actual registry hive) and give it the name "Software1"
As good practive always run this when you're done with edits:
REG UNLOAD HKLM\Software1
Indeed reg load, and reg unload were what I needed though I had problems saving it back to the image as it corrupted it, I'm sure I could of resolved that but I ended up using Jan's InjectMsd which I was advised is agnostic to the contenct of the .reg file...