Let´s assume that your internal network is 192.168.1.x.
Just add the following line at the end of your rc.local file:
/sbin/ipchains -A forward -s 192.168.1.0 -j MASQ
This will allow everyone inside your network to use NAT.
If you want to to enable only certain computers, add as many lines as the number of computers you want to enable, putting each computer IP in each line, as follows:
/sbin/ipchains -A forward -s 192.168.1.3 -j MASQ
/sbin/ipchains -A forward -s 192.168.1.5 -j MASQ
/sbin/ipchains -A forward -s 192.168.1.9 -j MASQ
This will only allow computers with IP´s 192.168.1.3 , 192.168.1.5 and 192.168.1.9 to use NAT.
(note: some prior versions of e-smith do not support the ipchains command so make sure you have e-smith 4.1.1 )
Hope that helps!
Jean Zouki Junior