Well, it's possible and easy to lower the pitch of a voice. Use Effects/Time and Pitch then select the "Stretch" option. Select the "Pitch Shift--Preserves Tempo" box and drag the control until you get the pitch you want.
The bad news, though, is that the degree of shift you want is going to sound very artificial and "science fictiony" rather than a real voice. If you want the reasons behind this, do some Googling on the word "formants". The human voice is very complex in how it produces sounds and simply playing with pitch makes the relationship between these sounds incorrect and artificial. I believe there are very expensive bits of software that can do this but I've never played with them.
The other parts of you question suggest to me that you may wish to attempt what is known as "looping" or "ADR" in the professional film industry. This is getting an actor to re-voice the dialogue that was recorded on location. In a big budget film, it's not uncommon for 80 or 90% of dialogue to be replaced this way. Basically, you need to break your dialogue into short (one line) segments and arrange your monitoring so the actor can hear it over and over (usually with two or three timed "beeps" counting down to the line) so the actor can get the timing and speak in unison with the pre-recorded audio. It's time consuming but not TOO difficult. However, be careful with the acoustic of where you're recording. An echoey room will sound wrong if your scene is shot outdoors for example.