this is the well-known drawback of using client nat.
If you need to know the original client ip for statistics, you should try to collect the stats before the CSS.
Otherwise, you will have to avoid client-nat and use policy routing in your network to guarantee that the response goes back to the CSS.
There is no way to insert the client-ip info in a non-HTTP protocol.
Moreover, the CSS can only insert the ip address in HTTPS requests and only if you have the SSL module.
A solution that may looks very easy [like client-nat] may sometimes come with important drawbacks [no info about original client ip].
Gilles.