How Do I Add A Pre-recorded Voice Over to an iMovie Project?

You can drag in the voiceover file and drop it on top of the clip where you want it to start. It does not have to be perfectly placed, because you can fine tune it later by dragging.

I would suggest putting your voiceover file in iTunes and dragging in from the music browser, but it also works to drag in from the Finder. If you drag in from the Finder, you must take care not to move it later, or the iMovie Project will not be able to find it.