Board Thread:Questions and Answers/@comment-5693502-20140410203934/@comment-4310945-20140416201136

I would agree with this as well. If you think about it, it is similarly possible for an Arrancar to use an asauchi to obtain a shikai and later bankai. But it's all just speculation at this point. Unless Kubo says anything about it one way or the other we'll likely never know.