User blog comment:B214/Question and Answers 4/@comment-24173056-20150224030239/@comment-16281343-20150224090823

Issei IS considering on adding them to his peerage. In v19, after Xenovia reminded Issei of her intent to join his peerage, Issei thought of him already filling three of the spots right after he gets his Evil Pieces.

When he takes them away from Rias is the question as I doubt Issei would want to hinder Rias' dream of becoming the Champion. He might not immediately take them away but there will be a time that they will be Issei's.