Need Help on UDF

Dear Experts,

I am working on File to File scenario. where i need to write UDF for genearting dynamic file name.

like this my source file name:sf_out*.txt

target filename:sf_out_*txt.pgp

i have gone through scn i found below code.

tring filename = new String("");

DynamicConfiguration conf1 = (DynamicConfiguration) container.getTransformationParameters().get(StreamTransformationConstants.DYNAMIC_CONFIGURATION);

DynamicConfigurationKey key1 = DynamicConfigurationKey.create("http:/"+"/sap.com/xi/XI/System/File","FileName");

filename = "xi_output_" + variablePart + ".xml";

conf1.put(key1,filename);

return variablePart;

where exactly i need to change the code as per requirement?

Thanks & Regards,

pavan