Search for existing user or perform XHR request to generate new user record from remote source

I have a “users” sheet that contains registered users which are referenced in other tables. I’d like the ability to have a field either autocomplete a user record based on email (like google admin does) from the sheet of existing users, OR, if the entered email is not found create a new user record based on the result of an API call to one of our services which would return a JSON object describing the user fields to be populated. Is this possible? Is there another pattern that would accomplish this in a simple (for the user) way?

There is no inherent support for this, but you might be able to kludge something together.