Hello there! Is there a way to upload or import multiple users via an excel document or something like that?

Nothing exists for this, you would have to create it. Take a look at this class: https://docs.zenphoto.org/1.5.x/class-Zenphoto_Administrator.html (Note it will be renamed in 1.6). The easiest would be to use a CSV file as there native PHP functions for this available.