User blog comment:Psy0Kryst/Masai Kurosaki in Soul Society/@comment-7092747-20130603163603/@comment-8295743-20130604022535

The soul merges with the collective within the Hollow. The only known Hollow who could use the power of the souls he devoured was Metastacia, a Hollow that Aizen created. There is also the Arrancar called Aaroniero who had the same ability. We don't know what happens when a Hollow is killed by a Soul Reaper, apart from knowing that it purifies the Hollow and it is allowed to enter Soul Society. Just one of a billion questions that Kubo has not answered.