And this from Veritas people:
Do not do this!
This is not a supported feature and I would highly advise than you not
attempt this on a production system. Rootdg was designed for just that, to
encapsulate root. If you really do not wish to encapsulate root then I
would advise finding an unused disk - not your boot- disk and creating a
rootdg disk group there. May I ask why do you not want to encapsulate root?
However, if you still insist. Here is the procedure:
#vxconfigd -m disable
#vxdctl init
#vxdg init rootdg
#vxdctl add disk c0t0d0s7 type=simple
#vxdisk -f init c0t0d0s7 type=simple
#vxdg adddisk c0t0d0s7
#vxdctl enable
#rm /etc/vx/reconfig.d/state/install-db