I wonder, since a soul is like the inner self of a beings identity, and the potential of one who has infinite souls, they could possess and develop several identities throughout lifetimes each with its own experience and skills. One could possibly even harness the skills of "past lives" souls and further refine them, with this process capable of happening indefinitely for every life an individual chooses to live as.
One might not "manipulate" these souls, but they can still influence them and connect with the souls to gain new experiences and learn from