ip route add 192.168.1.0/24 via 1.2.3.4 src 192.168.2.1
The network scripts are prepared for this. Create a file called /etc/sysconfig/network-scripts/route-<devicename> with content "192.168.1.0/24 via 1.2.3.4 src 192.168.2.1" (the ip route add is executed by the network script automatically). You can read about that in /usr/share/doc/initscripts*/sysconfig.txt.
ADDRESS0=192.168.1.0 NETMASK0=255.255.255.0 PREFIX=255.255.255.0 GATEWAY0=1.2.3.4