Thanks Gert.
I was looking for a "one click" solution within SME.
Next best is the add_drive_to_raid command. I'm not sure how it works with a 'hot' spare though. Time to play in a VM.
[root@tiger ~]# cat /sbin/e-smith/add_drive_to_raid
#!/usr/bin/perl -w
#----------------------------------------------------------------------
# add_drive_to_raid: Add spare disk to existing raid arrays
#----------------------------------------------------------------------
# Copyright (C) 2005 Gordon Rowell <gordonr@gormand.com.au>
# Copyright (C) 2006 Shad L. Lords <slords@mail.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
#----------------------------------------------------------------------