Over time, people's voices typically drop by about an octave from the effects of testosterone, so you'd have to be starting with an unusually low voice to end up with a baritone speaking voice. If you want to extend your low range beyond what T gives you, your best approach would be to work with a good voice coach and find out what's possible. There are DIY methods on YouTube and elsewhere, but some of them carry a real risk of damaging your voice.