How to add a source file to a executable that only exists after ExternalProject_Add in Cmake

I have a cmake project, where it is pulling an external project using ExternalProject_Add() and I don’t want to use it’s build files, but just would like to add its source files to either a add_executable or add_library call. The problem is when initially calling cmake, those source files don’t exist yet, so it fails with “Cannot find source file:” Is there a way to tell cmake to not check for their existence, or a better way to do this?

I’d say you’d either need to use FetchContent (as it does its job during configure step instead of during build like ExternalProject) or use a superbuild pattern.
CMake always checks if all the listed files exist or are known outputs of custom_command, etc.