You would need to create a microflow to combine first name and last name and populate that value into the full name attribute you'll add to the entity. This microflow could be called in one of 3 ways:
For this use case, 1 is the preferred option IMO, because this information will change infrequently so you are incurring the cost to update only when needed. 2 would update the full name every time an employee is committed, not wrong but incurring unneeded cost because the employee's first and last name won't change every time the employee is committed. 3 is the least preferred option because the microflow will execute every time an employee is retrieved, for instance when you view a list of employees, the microflow will execute for each employee, so 3 is the most costly option.
Hope that helps,
P.S. if you anticipate these employees logging in to your app, you should set them up as a specialization of System.User or Administration.Account as this will create these employees with the ability to login. This opens up a number of other things that need to be taken into consideration regarding user security and user creation. For a good example of this in action, download and view the Employee Directory app from the App store. It has an Employee entity with Full Name and an event handler that populates that attribute. It will also give you some examples of how to create users and manage user security. For an intro to some of these topics, you can read this how to: https://docs.mendix.com/howto6/create-a-secure-app