Hi Peter
AFAIK, the RFC adapter only supports RFC-XML message protocol.
You configure the receiver RFC adapter to convert XML messages from the Integration Server or the AEX to RFC calls and send them to the corresponding receivers.
Even the documentation mentions this explicitly.
Configuring the Receiver RFC Adapter - Advanced Adapter Engine - SAP Library
Why do you need to convert it to a flat file prior to it being processed by the RFC call? Looks like you are trying to create a flat file IDoc. Why don't you use the standard IDoc converter modules in a File receiver?
Rgds
Eng Swee