Context: existing stripe out of two disks, c0t1d0s0 and c0t5d0s0;
Adding disk c2t0d0s0 as concatenation to the existing stripe.

sipb-server-1.mit.edu# metadb 
        flags           first blk       block count
     a        u         16              1034            /dev/dsk/c0t1d0s0
     a        u         16              1034            /dev/dsk/c0t5d0s0

sipb-server-1.mit.edu# metastat
d0: Concat/Stripe
    Size: 8389730 blocks
    Stripe 0: (interlace: 128 blocks)
        Device              Start Block  Dbase
        c0t1d0s0                1090     Yes  
        c0t5d0s0                1090     Yes  

sipb-server-1.mit.edu# metadb -a c2t0d0s0
sipb-server-1.mit.edu# metadb 
        flags           first blk       block count
     a        u         16              1034            /dev/dsk/c0t1d0s0
     a        u         16              1034            /dev/dsk/c0t5d0s0
     a        u         16              1034            /dev/dsk/c2t0d0s0

sipb-server-1.mit.edu# metattach -i 64k d0 c2t0d0s0
d0: component is attached
sipb-server-1.mit.edu# metastat
d0: Concat/Stripe
    Size: 12585140 blocks
    Stripe 0: (interlace: 128 blocks)
        Device              Start Block  Dbase
        c0t1d0s0                1090     Yes  
        c0t5d0s0                1090     Yes  
    Stripe 1:
        Device              Start Block  Dbase
        c2t0d0s0                1090     Yes  

sipb-server-1.mit.edu# df -k /news
Filesystem            kbytes    used   avail capacity  Mounted on
/dev/md/dsk/d0       4127335       9 4086053     1%    /news

sipb-server-1.mit.edu# growfs -M /news /dev/md/rdsk/d0
/dev/md/rdsk/d0:        12585140 sectors in 11546 cylinders of 10 tracks, 109 sectors
        6145.1MB in 145 cyl groups (80 c/g, 42.58MB/g, 5440 i/g)
super-block backups (for fsck -F ufs -o b=#) at:
[...]

sipb-server-1.mit.edu# df -k /news
Filesystem            kbytes    used   avail capacity  Mounted on
/dev/md/dsk/d0       6191631       9 6150349     1%    /news