Yes, you can still make changes in iMovie even after saving your project.
Yes, you can still make changes in iMovie even after saving your project.
You can import it into a new iMovie project and add your voiceover, then export it once more, though the quality might not be great compared to using a headset mic.
Open Audacity, record your voice, then export the audio in the appropriate Mac format. Next, move the file into the video project.