You can use ws-outbound-gateway to make a webservice call whose output will go to the repolyChannel and downstream and I think that is what you want.
I'd suggest to look at our samples especially this one: https://github.com/SpringSource/spri...tbound-gateway
As you can see its a basic chain that has header-enricher to enrich a request with SOAP Action header, makes a SOAP request and the output goes to the output-channel which in this sample is logger but you can stick your service-activator and do whatever you want with it.
<chain input-channel="fahrenheitChannel" output-channel="celsiusChannel">
<!-- The response from the service is logged to the console. -->
Now, the example above is SOAP, you can do the same with REST, just look at 'http' sample