Creating user with a normal name but later associate a company email or free email like gmail yahoo

We are in process of implementing Leave Managment and HR, with payroll.

Many of our employees are not having company email or even email. So we have to create a user with normal name like JOHN_SMITH, RAJU_PATEL, etc,.

the question is how to assign/associate email-id to a user after creating employees and its related user with normal name. there can be different email scenario like company email or gmail/yahoo id.

can a dummy user name can be created and associated with a gmail account later ? How to associate/assign an company-email or gmail/yahoo email after creating user/employee with normal name like RAJU_PATEL so that notification etc can be received by user.