In order to function as extractors rather than ejectors, the ejectors must be replaced with a similar part that lacks the "hook" on the front.
Simpler solution (from somebody who has put a couple hundred thousand rounds through over/unders).... as you open the gun by moving the lever with your thumb and easing the forend down with the other hand, place the palm of your "lever hand" an inch behind the shells. They'll pop back, hit your palm, and stop there..... actually more accessible than with extractrs, which barely lift them out of the chamber.