When you add an executable file or script to Post-Setup, NTLite makes a copy of it to a staging folder on the ISO.
Normally that's under "\sources\$OEM$\$$\Setup". If you only need to run the executable or script once (and never again), then the file will disappear from the staging folder at the end of Post-Setup. You don't have to copy this file yourself to the ISO folder. NTLite does the work.