We just had a minor panic as our filer filled - the 'raid add' command is gone. For your reference, if you've moved to the new OS, here's the replacement command:
vol add vol0 1
This presumes you have one volume, which is the default. "vol status" will give you a list. This will add any spare disk to the volume - you can also specify which one to add if you'd like.
A handy trick if you run out of space is to reduce your snap reserve space by a few percent while the spare disk is added to the volume. Adding the disk takes a bit of time, so this space stolen from snapshots will keep you running.