Hi Markus,
Are you looking to provide a dynamic name to the flat file you create through IDOCXmlToFlatConvertor module for the IDoc received from ECC ?
AFAIK, you will need to do a Java mapping or simple graphical mapping if you want to use payload fields to generate the file name dynamically.
It is simpler than making a custom module and deploying it.
Hope it helps!
Ambrish