proxy for clients behind firewall

Our clients are behind a firewall with an address that is not valid to our company network. Our network has a dmz with webservers. these addresses are valid on our company network.
We are using MS Proxy 2.0 for NAT to be able to browse the companies extranet with our clients.
This extranet does not always has URL addressing but uses mostly company internal ip addresses.

Now we get the possibility to browse the internet through a proxy server which uses port 8080.
If we configure our proxyserver as upstream we cannot connect to the extranet or the intranet anymore.

Can someone tell me if it is possible to configure proxy server to use another proxy, but with the exception of for example 10.* addresses?