Smit tcpip, Further Configuration, Network Interfaces, Network Interface Selection, Configure Aliases, Add an IPV4 Network Alias, Select appropriate network interface:
en0 1L-08 Standard Ethernet Network Interface
then, enter alias IP address and network mask such as 255.255.255.0.
The interface en0 was selected above since that is the most common. You may be using en1 or en2, so select the one that is appropriate for your environment.