Below every schedule, there’s a separate section for as many extra shifts as you may need for a unique occasion. Simply click ‘create’ and add the shift as you would any other.
When you’ve saved it, an extra row will appear below the shift, so you can continue and add yet another shift, if need be.
If you need an extra row on a more regular basis, you can create an extra position.
Go to Settings > Schedule > Schedule positions and click 'create position' to permanently get an extra row to your schedule.