An interesting idea.
I personally believe that as long as they have an elaborate reasoning and storyline, its possible.
However, they will have their original powers WITHOUT the benefits of espada or their elite equivalent.
It might make more sense that if someone were to join as the shimigami or vaizard first, then roleplay for a while, and then role play a way onto the other side.
That way it'll make more sense.
I'll pass the idea by the other admins and we'll discuss it.