When creating a RAID 1, some systems will copy the data from the original drives onto the added drives, but you want to be certain that it will copy in the right direction. It would be unfortunate if it copied the new drives over the old.
I do not know what your particular motherboard will do. Even if it promises to do the right thing, however, I think it would be extremely foolish to count on it - a power failure at the wrong instant could really ruin your day.
You should get a large external drive (a 2TB would be good, but 1.5TB or 1TB might do if your system drive isn't full), and use something like Ghost to make a complete image of the existing system drive. Then add the drives.
If adding the drives works correctly, then you have a backup of your system as at the moment you did this - backups are GOOD!
If adding the drives messes up your data, then can restore the image to the created RAID array (so you lose nothing), and you still have a backup of your system as at the moment you did this - backups are GOOD!
Sounds like you win either way, doesn't it?
If you were feeling paranoid, you could always get a second external drive, and make TWO images of the system drive...