I think you have to start from scratch. The RAID user guide at http://www.intel.com/support/motherboards/server/sb/CS-022358.htm says "Caution:All data on the virtual drive is erased during an initialization."
Thanks for the info Dan. I thought i would have to start fresh with with the Raid 1 creation, but i have been able mirror port 1 from port 0 after creating the array on a promise hardware raid card. Different server though.
If you are running Linux, you can create a RAID-1 volume with only one drive, copy your data over to it, then add the data drive to the RAID-1 vol. This isn't hardware based, but it has saved me a lot of time and grief in the past