Yes, you can add new voice-overs by doing the following:
- Open the existing movie in the Player
- Select the frame where you want to start recording from
- Open the Sound menu
- Click Record Sound...
- Make sure the correct microphone source and then click Record.
Playback of the movie will then begin from the selected frame with anything you say into the mic being recorded. Recording will continue until playback is stopped.
If you only want to record voice-over for a specific section of the movie, make a selection of those frames instead of just selecting the first frame. Recording will automatically stop when you reach the end of the selection.