Dark energy

Yes, you can do it the same way you usually split your army, and you can split it in the window where you can see the available creatures.

You can also immediately add that if all the slots are empty, and it's better to raise a new unit than to leave an existing one, you can simply hold down the right mouse button on that unit and drag it to any free cell in the top row... Confirm the disbanding of the unit, and the slot will be free.