Is there some reason that the stopper (and the bottle, for that matter) are characters and not props?  I'd probably either make them both the same character, or parent the stopper (prop) to the bottle (character, or prop).

But either way, just...

1. load your prop/character and open the Joint Editor.
   2. select (click on) the actor (body part) in question.
2. set the option at the top of the Joint Editor to 'Center'
3. Note where the little green cross-hairs are... this is the point about which this actor will rotate....
4. click on it and drag it where you want it (use the 3 value input boxes for fine or absolute value adjustments).
5. save your prop/character back to the library.

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