you can do as dukenuke suggest which is a good safety method in which you create another bigger filesystem then ufsdump or tar the data from old filesystem to new filesystem.
cd /DATA
tar cvfp - * | (cd /mnt;tar xvfp -)
anything goes wrong later you still have the old filesystem to fall back upon. The not so pleasant thing abt this is you may need a long down time depending how much data you have to copy, at least few hours looking at the size of your current filesystem.
or you can try growfs
example from man pages.
d80 already contains 2 submirrors d9 and d10, add additional disk c0t2 and c0t3 to d9 and d10
# metattach d9 c0t2d0s5
# metattach d10 c0t3d0s5
# growfs -M /files /dev/md/rdsk/d80
check the man pages on how to do growfs, I think need to schedule some downtime to umount FS before you can do do a growfs. Make sure adequate backups are done prior to doing this.
I think man pages says you can do growfs online but a little extra 1-2hr of caution in a downtime is better than spending many sleepless hours trying to resolve a mistake if something went bad