We use employee numbers as usernames, but have the employees real name entered in the account description field.
If this field is accessible via a relationship, then you can use either the calculation or looked up value auto-enter options to copy the data into a text field in your new record. You can auto-enter the username into one field and use a second field to auto-enter the name. Or you can just link to that record in a related table.
But account names might be a better option than user names--which are specific to a given user account on a given computer. An employee could log in with their account name on a different employee's work station--perhaps during an emergency, and the work will be logged under the wrong employee ID if you use a user name.
Are your users entered in the filemaker security, LDAP, or in a database table?