How to convert an exiting partition to RAID on OS X?

Unfortunately you have to format everything first.

Easiest way will be to use Superduper! or similar software to make an image of your current setup on another disk, then create the RAID array and restore the image back.

Enabling a RAID is destructive to the existing data using disk utility and native OS X tools.

You will need to have a temporary copy of the data to re-load it onto the new RAID volume.

I know several people that always set up their externals to be in a RAID so they can always mirror up existing drives after the fact to duplicate, snapshot or extend/stripe them after they have data on them.

Never been worth the hassle for me to set things up that way, but it works long term for many.