Adding silences in mp3 file without converting

do it lossy with Audacity or Traverso-Daw:
http://traverso-daw.org

So you could use directmp3cut that works under wine:
http://mpesch3.de1.cc/mp3dc.html#dwn
Split lossless,
Create a silence mp3 with command line : http://stackoverflow.com/questions/5276253/create-a-silent-mp3-from-the-command-line

After that you can merge the mp3's lossless: http://lyncd.com/2011/03/lossless-combine-mp3s/