How to include an additional CMakeLists.txt

If this files just contains functions definitions you should not call it CMakeLists.txt, but some_adequate_name.cmake

Than you can include it where you need those functions definitions using include.

That can be done with include, here's some simple example:

Content of A/CMakeLists.txt

function(foo)
    message(STATUS "heya")
endfunction()

Content of B/CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
include(${CMAKE_CURRENT_SOURCE_DIR}/../A/CMakeLists.txt)
foo()

Now, including another CMakeLists.txt will also run everything in that file, so you may want to avoid doing that if there are targets in B/CMakeLists.txt

If you can modify your code, it would be better to define your functions in a "top level" CMakeLists.txt before calling add_subdirectory.