Grubby,
You know they make them there capos for a reason. We can't all sing in G, that's why there's all of those other keys.
I suspect you may be rather new to all of this. As you learn guitar, you will find that some songs work best in a specific key. Like you want to do Robert Johnson's "Love in Vain" with chords in a G formation if you want it to sound at all like Robert. But if that is too low, just capo up to where the pitch is comfortable.
As you work your way through transcriptions of the great blues and folk singers you will find that allmost all (I know of no exception) use a capo.
Now, given all that, you may be able to extend your vocal range with muscle relaxation, etc., but the advice of a good vocal teacher would be helpful to you.
Roger in Baltimore