To add a package to the registry, simply fork the repository, edit
manifest.json as needed, and send a pull request. Make sure
that the package you're adding has a package.json file.
As with contributions to the Alkahest core, packages that are borderline cheaty will not be accepted. If your package seems sketchy, you will need to make a strong case for its inclusion into the package registry, and even then, there is no guarantee that it will be included.
Please note that the core developers may sometimes remove a package from the registry if it has been broken for a long time with no fix in sight, or if it has received sketchy functionality after being added to the registry.