First of all you can place a request message into headers, using a Header Enricher endpoint: https://docs.spring.io/spring-integration/reference/html/message-transformation.html#header-enricher.
Then after receiving response and following downstream, you can extract that original message from those headers.
Second: you may consider to use a Content Enricher instead of general service activator: https://docs.spring.io/spring-integration/reference/html/message-transformation.html#payload-enricher. So, you will be able to keep an original payload and extend it to the reply content.
And lastly: you can do whatever you need inside a custom service activator method. So, you receive a message there, send request, get a response and combine both of them together to return some reply message.