Is there a way for me to add/create a new site in my Firebase project in the same way I deploy new projects using the CLI.
The Firebase CLI has a firebase hosting:sites:create
command that looks promising. It looks like that wraps the corresponding /projects.sites/create
REST API.
What doesn't seem possible at the moment (according to the answers to this question) is to add a custom domain through the CLI or REST API. That is currently only possible through the Firebase console.