Those headers are added by kamailio only when it is the origin of the request (adding user-agent) or the response (adding server). If kamailio is forwarding the request or response, it doesn't add any of these headers.
You can add them using append_hf(...) in the request_route{} or reply_route{}.
I am going to close this issue, if you need to seek further clarifications, use the sr-users@lists.kamailio.org for discussions.