The User-Agent HTTP header includes a single value, and is not designed to include additional details. However, you can use policy substitution to maintain the original User-Agent value and append new values. The substitution string appears as follows: $(request.header.User-Agent)
$(user) and $(client.address) are what you want to append to the header.
define action ModifyUserAgent
set(request.header.User-Agent, "$(request.header.User-Agent); $(user); $(client.address) ")
end action ModifyUserAgent