Using CPM.cmake in CMake scripts

[itavero]

Some background:
We had a project in which we have created several CMake modules. Now I want to take these CMake modules out and reuse them in other projects (also see #390).
This works up to a point, but we also have a few CMake modules that also have a CMake script (which we execute via a add_custom_target or add_custom_command).

Now the problem arises is that one of those scripts depends on the another CMake module, which I now want to include via CPM.cmake.

Unfortunately, at this time, you can't use CPM.cmake from a CMake script as it uses add_subdirectory, which results in the following error:

[build] CMake Error at C:/cpm-cache/cpm/CPM_0.35.5.cmake:938 (add_subdirectory):
[build]   add_subdirectory command is not scriptable

What would be the suggested way of dealing with this situation?
E.g. when a CMake scripts needs to pull in a "module" via CPM.cmake, how can it be achieved?

[itavero]

A workaround (besides adjusting CPM.cmake) would be to already CPMAddPackage the module A in the CMake file of module B, which is included in the project and then passing the path to A to the script that belongs to B.