Perhaps I'm not understanding what you mean...but the only way you can lower pitch and stay in the same key is to lower it by a full octave. And that's likely to sound more than slightly odd.
The term is "formant shifting"; the formant in your voice is (this is a terrible explanation) what makes you sound like you and me sound like me, giving us each a distinct timbre.
If you put ReaPitch on the track, turn the Dry all the way down and the Wet to 0dB, you can use the Formant sliders to play with the sound of your voice.
(Obviously it'll sound worse the farther you try to go)