How could you add SA, for example, without being the original licensor, for importing to Wikipedia?
The SA license would apply to the derivative work. The non-SA license would apply to the original work. You aren't "adding SA", you're creating a new work, and licensing that new work under SA.
That's my understanding as well. There's nothing stopping a bit of content being available under multiple licenses. In this case it's available under CC-BY as a stand-alone work and GFDL as part of a large work. I don't see any problem with that. There's nothing stopping people using the original work under the original license just because you've used it under a more restrictive one.