Setting up RAID1 with drives that already have data on them. Also need dual enclosure

I have a pair of 2TB drives in my Mac Pro that are mirrored manually with ChronoSync. I am going to reitre the Mac Pro and want to put those drives in a dual drive enclosure.

I don't mind manually mirroring the drives with ChronoSync if the external enclosure allows both drives to be mounted separately. But also, if a Raid1 would work just as well, I could do that too.

Problem is, the 2TB drives are both loaded with data, and I have no idea if I can create a Raid1 out of that. I could erase one drive and place both in the enclosure, and then run the Raid1 setup?

I've been looking at this drive: http://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/MirrorRAID/ which can be configured as a Raid1, though it would ship as Raid0 (no idea how I'd change that). Are there other enclosures that are more highly recommended?

I currently do not have a USB3 computer, though if I get a new Mac Pro that will have USB3 and TB2. This drive is for archival data storage and itunes video serving only, so FW800 is fine for that.

What are my best options here? And, when setting up a Raid1, how do I handle the drives already having data on them?