The Things Stack uses frequency plans from the lorawan-frequency-plans Github repository.
To add frequency plans, you may open a pull request on the lorawan-frequency-plans Github repository, which makes the plan available in most deployments when the next version is deployed.
You may also fork the repository and reference your fork, with potentially custom plans, or with standard plans left out.
You may also configure a local directory (or cloud storage bucket) where the frequency plans are stored, in case the private network cannot access GitHub or does not want to rely on GitHub.
See configuration options to configure a different repository or directory for frequency plans.