Similar as on FreeBSD and Linux, you can add/remove static network routes through the command line with the route command. The syntax somewhat differs from Linux, though.
To add a route (e.g. subnet 192.168.3.0/24 to gateway 192.168.1.254):
sudo route -n add -net 192.168.3.0/24 192.168.1.254
To remove a route:
sudo route -n delete -net 192.168.3.0/24
To show the current route table, you can use netstat: