1) Yes you can convert an existing partition into a RAID 1 array. The basic steps are:
-Create a single-disk RAID-1 with your new disk
-Copy/move all your data from the old-disk to the new RAID-1 array
-Verify the data move was successful
-Wipe the old disk
-Add the old disk to the new RAID-1 array
-Example at
http://wiki.archlinux.org/index.php/...system_to_RAID ; its for ext2 FS, so adopt it according to xfs
2) If a drive goes down in a RAID1 array, it will be marked as dirty. You will still be able to access the array. You can also remove a drive from the system and mount it on another computer. See the software raid how-to simulate a failure.
http://tldp.org/HOWTO/Software-RAID-HOWTO-6.html
I suggest you experiment with creating an array with two small partitions, and simulating a failure before you attempt to do this on real data.
Cheers