"Add" button will make the new option more meaningful, but it will ruin the traditional purpose of Contact List Clean-Up feature, because it helps not to clean but to make list more crowded. OK OK no problem
.
But the problem now is, what will happen when we press "Add" button after selecting a contact we already have
. Nothing will happen, will it? So since those contacts who we have are already being displayed in the first traditional option, do we really need to see them in both lists? I know, they must be displayed due to consistency but "Add" button is the problem here.
If I guess correct, you are tired and don't want to work on an Allow&Block lists cleaning feature for the next version. But if I'm wrong, if you are still interested in this feature, that "Add" button can be placed on my feature picture. Although that "Add" button may be invalid for some contacts again as stated above, yet it would be a good option for users.