Can you add an external fingerprint reader to a MacBook that will work like the built-in reader?
No, there's no external fingerprint reader that works like the built-in reader. The built-in reader is not communicating directly with macOS on the main CPU, instead it communicates with a seperate kernel (still Darwin) running on a secondary ARM CPU.

As there's no way to connect an external fingerprint reader to the secondary ARM CPU, it is simply not possible for 3rd parties to create something that works exactly like the built-in reader and offers the same functionality.