i have a number of songs in itunes and i would like to add 5 seconds of silence to the beginning of the song track. is there any way to do this?

If what you want to do is have 5 seconds of silence between tracks on a CD, you can set that in itunes under itunes/preferences/burning. You can set the gap between songs to be from 0 to 5 seconds, as long as your CD burner supports it.
If you want to actually add silence to the beginning of the file, you'll find the song files following this path: (hard drive)/users/(user name)/documents/itunes/itunes music
From there, you can import the song into your editor, adding the silence and bouncing to disk as an MP3 or even AIFF or WAV if you want.
Without knowing what editor you're using, that is as detailed as I can get.