how to include one drush make.yml file in another?
Edit: I do see that you were using the wrong keyword, it should be includes and not imports. 

It looks like you have the correct syntax above I believe. We use something like the code below in our make process, there are three different core makes that we call that one in, local, development, and build (we wanted to get updated versions in local and more stable ones in dev and build, just so we can update when appropriate.). It works pretty well but see the code below for an example. Thanks!

api: 2
core: X.x

defaults:
  projects:
    subdir: "contrib"

includes:
  - drupal.make.yml
  - libraries.make.yml
  - modules.make.yml
  - themes.make.yml
  - custom.make.yml


Here is the link for the Drush documentation for the make files, I didn't notice that they had swapped over to YML in the make docs or I would have started with this probably: Drush Docs - Make.