Short answer: Yes, you can use external camera input in Restream Studio.
Longer answer: Make sure the camera you would like to use is recognized as a web camera.
You can do the following test to check it:
From this page, see if your camera is listed on the “Select camera(s) below” box.
If so, then this particular camera will work with Studio.
On the other hand, if your camera does not show up on the above page, you can get your computer to recognize your camera as a webcam by using capture cards like the Elgato Camlink.
If you are on Windows and have a Canon camera, you can use their webcam utility program.