no, use the entity Administration.Account instead. This one is especially available for these kind situations.
Also; if you want to extend it with data which is used in the app process by other users; PLEASE create your own entity “Employee, customer, WhatTypeOfPersonItRepresents” and adda a 1-1 association between them. Never use your Account(User) object in your business process.
It’s not possible to edit the System.User entity.
But you can edit the Administration.Account entity (specialization of User) of you can create your own entity that is a specialization of System.User.