I could be all wrong here, but your question (to me) reads like the definitions of polymorphism. That is to say you want an A structure but you wand to mash it into a B object. Isn't that how you exploit Inheritance Polymorphism?

In otherwords It seems to me that even though it

. . .somehow feels wrong

your 2^nd options is the correct one. But my opinion is not from practice on such a problem as yet.