I believe the instructions on that sticky also apply to your situation. If you insert an unformatted HDD, you should be provided with an option to configure the new HDD as RAID 1.
Make sure you backup your data to another physical storage device before performing this procedure in case the procedure fails. Additionally, RAID 1 is not a backup, so if you're data is valuable, you should create a backup anyway to protect your data.
Please post your results.