npm submodule <pkg>
DESCRIPTIONIf the specified package has a git repository url in its package.json description, then this command will add it as a git submodule at node_modules/<pkg name>.
This is a convenience only. From then on, it's up to you to manage updates by using the appropriate git commands. npm will stubbornly refuse to update, modify, or remove anything with a .git subfolder in it.
This command also does not install missing dependencies, if the package does not include them in its git repository. If npm ls reports that things are missing, you can either install, link, or submodule them yourself, or you can do npm explore <pkgname> -- npm install to install the dependencies into the submodule folder.