chdev -l inet0 -a route=net,-netmask,<your-mask-here>,-static,<net address here>,<Gateway here>
For example to create a static route for destination 10.0.0 through 192.168.0.1
chdev -l inet0 -a route=net,-netmask,255.255.255.0,-static, 10.0.0.0, 192.168.0.1