What you should develop is an internal (canonical) message format. You transform to this format on the way in. The all your internal services can interact with the message. Then on the way out you transform it back to a format the destination can consume. A hub-spoke model. This will keep the amount of transformations you need to maintain to a minimum as time goes on.
Is there any way to expose the entire Message as an XML Document, or any plans to offer a feature like that?