When you create a RAID array, it needs to be initialized. Initialization will delete all data on a drive(s).
You can back up your OS and deploy it to the new RAID array. However, if you've kept your OS seperate from the data partition, you almost don't need to. Before you commit to backing up and deploying, verify that your backup solution wont get snagged on the RAID level. It would be a real pita if you expected to deploy your OS to RAID 1 and your backup software didn't play nice.