If I understood it correctly, you would like a directory copy and manually change the files on the ISO/USB, or you want a command that runs something from a partition on a fixed drive of that machine?
In the case of first, to copy a directory add some to the image and change its Mode to External (ISO), and choose final destination on the right.
That way it will create a directory under the ISO \Sources\$OEM$\... which gets deployed where instructed.
Find it there on the ISO/USB and change files manually - no need to edit WIM image.
If you wanted a command, you can add commands, but it's tricky to guess fixed drive partition letter, needs some testing or a for loop.