Like the others are already saying, this is kind of impossible to do, due to the end-to-end encryption. Other possibility would be to create your own messaging app which supports your idea, but I think the biggest problem would be to get people using it. There will be some users who are going to use your app but most will just stick to what they already have - lazy, easy, all friends are already there. Unless you are able to convince Facebook to do sth else