There is no such configuration parameter for changing the filename when you use filepolling mechanism.This is the default behaviour coded internally by WM.
Other option would be write your own polling routine and schedule it. The PSUtilities package has a listFiles service that returns the list of files (among other things). pub.file:getFile can be used to read in the data. and use moveFile(Complete/Archive directory or Error directory) and also there is deleteFile service available in PSUtilities package(you can get this from webMethods Advantage website).