Someone

// foo/bar/CMakeLists.txt
SET(FILES blah.cpp)
FOREACH(file ${FILES})
	SET(OUTPUT_FILES ${OUTPUT_FILES} bar/${file})
ENDFOREACH(file)

SET(BAR_SRC ${OUTPUT_FILES} PARENT_SCOPE)

// foo/CMakeLists.txt
ADD_SUBDIRECTORY(bar)

SET(FILES ${BAR_SRC} blah2.cpp)
FOREACH(file ${FILES})
	SET(OUTPUT_FILES ${OUTPUT_FILES} foo/${file})
ENDFOREACH(file)

SET(FOO_SRC ${OUTPUT_FILES} PARENT_SCOPE)

// ./CMakeLists.txt
SET(BASE_SRC ${FOO_SRC} /* etc */)
ADD_EXECUTABLE(myExecutable ${BASE_SRC})