Can I add my own environment key/value using (set-env! ...)?

Or can I only update the keys as referenced here https://github.com/boot-clj/boot/wiki/Boot-Environment

@mario.cordova.862 you can use your own keys if you want. underneath its an atom

Thanks!

Question: locally I can view the environment variables that were generated by doing cat .lein-env. Is there an equivalent of that using boot?