At some point you may have a solaris non-global zone up and running but are not able to take it down to add a network interface. For the longest time I thought in order to make any config change to a local zone would require the zone to be halted. Not the case any more with Solaris 10.
If you are wanting to add a permanent network interface to the zone, you still must use zonecfg to configure the zone so that once it does reboot the interface will be there. From the global zone do: