To create an employee in the system you need to either use migration template or through data replication or manually in the system from a existing Business Partner.
As far as I know, even if you create using migration template or through data replication, it will create a Business Partner in the system with role "Employee".
This could be the reason why it is not possible to create an employee from Employee BO directly. If you want to create an employee from existing Business Partner then you can use the action "CreateEmployeeFromExistingBusinessPartner" which is available in Employee BO.
Hope this is helpful.
Best regards,
Hari