Allow adding files from git submodule to parent git repo if they are in .gitignore in the submodule

Git submodules are pretty often used to "import" some external git
repository into your git repository, to not manage its files in your
git tree, that's good.

But very often we need to add some file to that submodule and manage
it in our local git repository, and this file is usually in .gitignore
in the submodule's git.

So, the example task is:

Copy a mysubmodule/config.example.yaml to mysubmodule/config.yaml
(which is in .gitignore in the mysubmodule), make local changes, and
store it in the local (parent) root git repo.

But the problem is that we can't add this mysubmodule/config.yaml file
to our root git repository, because receiving an error:

$ git add -f mysubmodule/config.yaml
fatal: Pathspec 'mysubmodule/config.yaml' is in submodule 'mysubmodule'

But I see no problems with doing this, if the file is in .gitignore of
the mysubmodule submodule.

So, let's discuss the approach of how we can allow this in git?

Or will be glad to hear and discuss alternatives and workarounds for this task.