You explained that perfectly well. I'm not using registrations much, but to my knowledge, that's not possible.
I can only imagine this could be done only within a style, when switching from one variation to another (by using complex fill-in). That still wouldn't be real crossfade, but could sound like it. And of course it would also depend on style itself.. some simple rhythm could be crossfaded that way, I think. I never tried that, though.
Bogdan