If the author of a Free software package (i.e. Artistic License, BSD or GPL) doesn't respond to correspondence, then you are certainly ethically able to do what you want.
My primary suggestion is change the name from MP3::(registered trademark of a money-grubbing corporation)to MP3::Opennap and release it under that name.
Added Later: Based on information supplied by others in this thread, I would definitely encourage patience with the author of the original package. I still think I would attempt to get a new name for the package with added functionality-- whether as a standalone or as a subclass.
Comment on (ichimunki) re: a question of utility and ethics...
This certainly seems like the best way to go. This means that the MP3::Napster module retains its vanilla features (i.e., not supporting the extended features of opennap), and we all gain access to the 'extra stuff.' Okay, I've got a copy of Conway's book, I guess its time to start reading that sucker a little more closely. Any suggestions on how to get some free beta testing? :)