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