I don’t think it will be possible to change to white screen to a green screen background. Chroma key removes a particular color, in most cases, the specific green color in a video, which is not a color often found in nature. The background screen can be other colors too, but you want it different from colors of a spokesperson i.e. the natural skin color, the shirt, hair etc which means yellow, white, black background are generally not suitable.
You can use the chroma key filter to remove white, but it will also remove any white part in the video eg white parts of the eyeballs, teeth etc and you’ll end up with something rather strange
If you can’t re-shoot the videos, one option would be to crop your spokesperson, then put that in a picture-in-picture in a small window or corner of the video.