It is not possible to use the Switcher Studio app on a camera other than built-in cameras on compatible iOS devices.
For DSLR cameras that allow tethering to a Mac, it's technically possible to connect the camera to a Mac and then display the camera view on your computer. You could then use Switcher Cast to show the computer screen in Switcher Studio. This solution is not ideal for most customers, as there is usually a delay and reduced quality. Switcher Cast also does not transfer audio.