My apologies. I went back and re-read what you were asking for so ignore my previous comment.
Hopefully these two screen shots will illustrate how easy it is to do what you want.
In this example I have a stick figure in front of a CK (chromakey) screen that’s too small for the frame. I CK him, the CK background disappears and he’s standing in the full frame.
Is this what you want to do?

