Merging global group_vars

How can I merge global in-memory group_vars with some other yml files?

We will have a best practice approch with a couple of groups and some corresponding roles.
The folder structure will have a top-level group_vars and host_vars folder. They get polulated through a custom inventory module that reads data from an external api.
We would like to then to merge the group_vars with local custom_vars. The merged values should be again in the global in-memory inventory.
Is there any existing module for that?

cheers
mario