Depending on your need you could easily use CSJS or SSJS for this.
 
create an onChange event of the Employee ID field with the script:
 
SSJS: 
 
getComponent("username").setValue(getComponent("employeeID").getValue());
 
 
CSJS: 
 
dojo.byId('#{id:userName}').innerHTML      (or  .value)     = dojo.byId('#{id:employeeID}').innerHTML     (or .value)