Only employees registered as shift workers can be added to rotations. However, you can enable shift work to your regular employees traditionally working on normal hours by following these steps:
Go to My employees
Select your employee from the list by clicking on view
Click on Shift settings
Click on Configure now
Populate the required information
Select a way to pay for overtime
Click on Save
Notice that this employee can now participate in your rotas, but their salary will not vary. It will remain unchanged.