This has a detailed explanation of software RAID:
To the best of my knowledge, you have to create RAID sets before putting any data on them. You can't just put in another harddrive and have it mirror an existing drive that you are using.
If you don't have a whole lot of stuff on there you want to save... the easiest thing to do is just reinstall and create the RAID set at install time. If that is not an option for you.. there are some more complicated methods that you can use.
Let me know if you need info on those.. but be forewarned.. they are not easy to pull off.