Assign Person to All Items in Board

Does anyone know how I could assign a person to all the items in a board through an automation?

Use Case: I’m duplicating an onboarding task list board for a New Hire. I want to automate assigning all the items in the board to the New Hire and their Supervisor.

That’s not quite what I need. The board is already created, so ‘When an item is created’ trigger won’t work.

I want to assign all the items in the board to a certain person - in a single action.