CMake/Tests/RunCMake/export/AppendExport.cmake

add_library(foo empty.cpp)
export(TARGETS foo FILE "${CMAKE_CURRENT_BINARY_DIR}/foo.cmake")
install(TARGETS foo EXPORT fooExport
  RUNTIME DESTINATION bin
  LIBRARY DESTINATION lib
  ARCHIVE DESTINATION lib
)
export(EXPORT fooExport APPEND FILE "${CMAKE_CURRENT_BINARY_DIR}/foo.cmake")