You can only assign tasks to participants using their email address because that is their username. So you could do a database lookup when the name is selected on the form to pull back the employee's email address so it can be used to assign the user task.

If you have a mix of named users and participant users, you can take both values and assign them to the task. You will see errors show up in your logs on the server that user x could not be found, because only one of the values would be valid for a user name, but you can ignore them.