Can one add further source files to an executable once defined?

Use target_sources, available since cmake 3.1

eg. target_sources(MyExampleApp PRIVATE ${extra_file})

https://cmake.org/cmake/help/v3.1/command/target_sources.html

It should be noted that for more recent versions of CMake (> 3.1 I think) one can append files to the SOURCES property on targets.

http://www.cmake.org/cmake/help/v3.3/prop_tgt/SOURCES.html

I think you may use:

add_executable(MyExampleApp main.cpp)
add_library(library STATIC ${ADDITIONAL_SOURCES})
set_target_properties(library PROPERTIES
     LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
target_link_libraries(MyExampleApp library)