The Definitive Guide to Data Quality
I had exactly the same requirement in the job I have been working on recently.
I resolved it as suggested by Sabrina; before passing my data to the tSoap, I used a tSetGlobalVar to record the information that I wanted to preserve until after the tSoap. I then used a tMap to bring that data back into the flow.
It's not ideal; it would be nice if tSoap had some pass-through capabilities so that unused values in the "input" schema could be included in the "output" schema.