There are multiple ways to skin this cat, as others have suggested. None of them involve adding an additional switcher on the stage. Unless your actual goal here is to create a Rube Goldberg machine.
torinth wrote:I am trying to figure out a way to install multiple cameras on stage
How many cameras are we talking? What type of cameras? (eg. operated locally, PTZ, fixed...)
torinth wrote:I have inherited a setup with a black magic atem extreme iso
What do you use this setup for? Streaming? Recording? ISO recording? IMAG?
What inputs/outputs are there other than the cameras on stage?
The answers to these questions will help inform the best solution.