Officially, yes, you need to off-load to a backup, configure the RAID-1, and reload onto the mirrorset.
If you're feeling lucky, you can potentially [activate RAID-1 without the archive|http://www.afp548.com/article.php?story=20040827122302975]; on-line. Search for (other) discussions of using the DiskUtil tool and enabling disk mirroring for various of the previous discussions.
But if something goes wrong with the on-line conversion, a bug or a power drop-out or a disk glitch or or... ?
RAID-0 is striping; two disks together presented as one bigger disk; targets I/O performance.
RAID-1 is mirroring; two disks together presented as one; targets reliability.
RAID-0 and RAID-1 (and the other RAID levels) can be hardware-based or software based.
The Mac Mini Server uses software-based RAID.
And for completeness, neither RAID-0 nor RAID-1 is an archival strategy.
As for the other question, do you have one disk showing, or two? If two, you're not running RAID-1. All of the Mac Mini Server boxes I've seen have arrived
not configured for RAID-1. They've arrived as two single disks; as what the RAID folks tend to call JBOD; Just a Bunch Of Disks.