First off, beware - there are two documents on apple.com, both titled “RAID Utility User Guide - Instructions for setting up RAID volumes on a computer with a Mac Pro RAID Card or Xserve RAID Card” (here and here.
The first was created in 2007, the second in 2009. I would assume the latter is more relevant. On page 10 of that document we find the following statement:
Expanding a Volume
You can use the command line to expand (add space to) an existing volume from the available space on the RAID set that hosts it. Expanding a volume doesn’t affect data already on the volume To expand a volume: Open Terminal and enter the following command:
$ sudo raidutil modify volume –expand -n volume -s size
where volume is the name of the volume as shown by the raidutil list volumeinfo command and size is the amount of space you want to add to the volume.
Having been tasked with reorganizing some storage in a late 2008 Xserve, I thought this is perfect - I can resize my volumes without having to format/restore the whole server. In the worst case scenario, maybe having to restore the boot partition, but that's fine. So, boot up the installer, Utilities > Terminal, and:
# raidutil modify volume --expand -n R1V2 -s 1.6TB Unsupported by this version of the hardware/firmware.
Whatever. You kinda get used to this sorta crap dealing with Apple's servers. Of course, they don't have to explain to my users why the server upgrade's gonna take 3x more time than planned. This is the reason you should never give any time estimate on how long an upgrade will take - this gig just went from a few hours to an entire day, just due to this tiny detail.
Maybe they'll fix this some day (or just withdraw the instructions), I don't know, but it's something to be aware of, for sure.