I'm very certain that IPcop doesn't provide any way for someone in the external network to detect the private addresses of hosts in the local network (that wouldn't make any sense). Can you post a link to the article where you read this?
As for how exactly you can configure IPcop to do Source NAT, I'm not familiar with IPcop's web interface, and I can't find it at the documentation
(perhaps they don't mention it because it's too obvious or perhaps they use different terminology), however you can always configure IPtables directly to do NAT as described in the NAT HowTo
at the Netfilter's documentation. To do that, you must login at the IPcop box using SSH, as described here
Destination NAT is covered at IPcop's documentation as "Port Forwarding".