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

Question

Given I have defined an executable with its main source file in a CMakeList.txt file:

ADD_EXECUTABLE(MyExampleApp main.cpp)

Can I

Answer 1

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