I have done this in linux but not specifically sme.
At a command line type /sbin/ifconfig eth0:0 192.168.0.150 (your-ipaddress-here) netmask 255.255.255.0 up to test it works and assuming eth0 is your internal n/w card. This wont survive a reboot. Some people just put this command in the the boot script rc.local but better to add it to /etc/sysconfig/network-scripts.
Copy your existing ifcfg-eth0 to ifcfg-eth0:0 and edit to change the Device to Device=eth0:0 and your new ip address.
./ifup eth0:0 to get it up.
This takes no account of the sme templating system and probably the new script should be copied to /etc/e-smith/templates/custom/etc/sysconfig/networkscripts but others may need to advise you of this.
Good luck
Brian