As APM said.
When you already have RAID, depending on the RAID type, you can simply add the drive, but changing a non-raid to a riad drive will wipe it.
Such a shame cos I have also wanted to do that.
My irritating daughter has a full 60GB SSD with about 4MB left on it, and even though I have setup her personal files to the HD ( that has 300GB left on it ) and I want to give her another 60GB to make it up, I will have to wait until she says ok and I will rraid them then, but until that time, its just a blank disk doing nothing!