You can have an npm module with any arbitrary licensing.

https://docs.npmjs.com/files/package.json#license

No because how the hell would they know?

I author a module for my employer and describe it with their developers@ email and as UNLICENSED; how does NPM discover that I am legally authorized to use and modify this package?

As in it checks the license in the source repo and prevents installation of dependencies with licenses that prohibit source modification?