User blog comment:Timeoin/Ask Us Anything - Variety Pack 8/@comment-8674030-20131010141055/@comment-7153552-20131010152354

Soul trapping, and I am simply guessing here, will not be allowed on any player-character. They may explain this away via the Souless One bit of lore, or they may just outright forbid for system and game mechanic reasons. Either way, I don't see it being viable. They may allow black soul gems and stealing of those greater souls via the NPCs, but that's a stretch with as little as we know about finite mechanics.