Drupal 8, override User Class for a custom added field
Don't. The field system doesn't work like that, there is currently no useful way to get autocomplete.

You could replace the entity class with your own and add methods for your fields, but it's not worth the trouble IMHO as you might get into trouble of some other module wants to replace the class as well.

You could add helper method or functions somewhere else and pass in the entity.