I don't think you could transplant or even create a mouth without  pulling it into a modeler and doing some major surgery on the object. It might be easier to paint different mouth shapes on the texture since it's mouth is painted on anyway. If you have Poser 5 or 6 you could add a displacement map to simulate lips and indent the space for the mouth between the lips.

It's all fun and games...
Until the flying monkeys attack!!!