Hello
You ca use a UDF with a parameter for your filed B.
Then you need not to map B
sometihing like this:
DynamicConfiguration conf = (DynamicConfiguration) container.getTransformationParameters().get(StreamTransformationConstants.DYNAMIC_CONFIGURATION);
DynamicConfigurationKey key = DynamicConfigurationKey.create("http://sap.com/xi/XI/System/File", "FileName");
string filename="";
filename="nameFieldB";
conf.put(key,filename);