So you currently have two single drives and you want to add a third drive and set up RAID5 on the fly? I think that's going to be pretty much impossible.
You might be able to use something like Ghost or Acronis to copy partitions off the current drives, then create a RAID5 array and put those partitions back. You'd need something like BartPE to do it though -- neither tool deals well with RAID arrays on its own. Then again, it might not work either, and the only point at which you'll know for sure is after you effectively wipe all the hard drives... Even if you install the ServeRAID drivers before you take the images, you'll still need to repair the boot bits.
It would probably be simpler to look into adding two hard drives to each system and setting up a pair of RAID1 arrays. I'm still not 100% sure how you'd go about it with any certainty though; replacing a hard drive controller is pretty major.