Hi Martin,
It sounds like what you are looking for is a synchronous call so you get the response back from the target system. If the target system offers a web service, you could write custom java code in Sourcing (deployed as a custom jar) that can make a web service call. Please note, this will be all custom and it can be fairly complex.
Sourcing 10.0 which is currently in ramp-up supports inbound RESTful web services. I think it will make it relatively easier to accomplish what you are trying to do. We are targeting end of the year for Sourcing 10.0 GA.
Regards,
Vikram