I think you are saying not only do you want to keep the data but also keep the whole system setup. Not sure how to do that...however..
If I could back the whole drive up on a third I would just start over and then move my config files and data over after done.
Hope this tutorial helps, this is how I figured out how to do a raid 1. But does not cover your exact issue, but since you are new like me, maybe this will help you. http://www.bobhoffman.com/wordpress/?page_id=44
Good luck. Sometimes it is just quiker to start over.