You will need to write a .Net component that you can call from an orchestration, passing an XlangMessage and returning an XlangMessage.
In your .Net code you would read the data stream from the message and transform into a flat file stream
I have thought of another method of doing this, a custom assembler that would sit in the send pipeline. You would use similar code as above to convert the message data stream from xml to flat file. If you are dealing with large messages this may be more efficient.
What is the reason why you cannot use a flat file schema and why you are using an untyped xml message?
if you could explain a bit more about the process there may be other options available