I can't remember how this is done. If you want to be able to sing in a lower octave or pitch (?) do you tune the guitar up or down? I'm having a brain freeze and can't remember. I don't like using a capo so that is out of the question. So thanks in advance :)
If I have understood correctly your question, you should tune your guitar down... Many bands do this live in order to allow the singer to survive all the concert without dying... One semi-tone lower or a whole tone should be enough (it depends on your kind of voice in relation to the song you're gonna sing).
Black Sabbath tuned the low E down to a C# for making Ozzy's voice even higher than it actually was (!!!), but it was more a stylish trick than actually an effective need...
I hope I was helpful to you, M@tt...
Keep on rockin'
Albyboy