Update 7z archive with contents of an unextracted

问题:

In the 7z UI I can open 2 .7z archives, and drag and drop the contents of one archive to another and it's performed in one step. I've tried running the 7z u command like this, >7z.exe u C:\test.7z C:\NewZip.7z\TestDir\TestFiles\ as a basic example. The output I get is 'The system cannot find the path specified'

I understand that it's another step to just extract it and then update the archive, I would like to move it to 1 step rather than 2, especially when the UI handles it in 1.