The only way to this is to mirror the data drive to a third drive other than 1 of the 2 you will putting into RAID. After you have the array built, you can then mirror the image from the 3rd drive to the RAID array.
Acronis will do this quite handily.
The only way to keep your old data with any resemblance of security is to get a backup drive that doesn't sit in your computer (which reduces the possibility of loss of data by virus, theft or whathaveyou).
I highly suggest you buy an external drive if you want to sleep at night knowing your data is safer