No, and there would be no advantage. You cannot add a second drive and magically create a RAID. You have to configure at least two drives together for RAID, and they would both need to be in the exact same state, blank. As far as adding a SSD later for caching... I don't know but imagine that can be done without loosing the data. At worst you would have to clone the drive before installing the SSD.
My understanding is that RAID mode is needed for SRT caching to an SSD, and that you'll lose everything on your HDD if you change from ACHI to RAID when setting up SRT. Possibly there's a registry hack to work around this; but I thought I could maybe "future proof" myself by setting PCH SATA Control Mode to RAID now, even though I won't need it until later. I guess I'll just go with ACHI now, though, and worry about a solution if an when I need to change it.
Thanks for your reply.