There are a number of ways to record a VoiceOver into an iMovie. Recently I've seen students record directly into their iPad, using app like Quickvoice, the emailing the file to themselves to drop into their movie. I've also seen students taking MacBooks into a quiet room and recording directly into iMovie (or even using QuickTime or GarageBand). Today, Year 5 were putting a short commentary into a movie and did it directly into the iMac. They found that if you speak quietly, up close to the mic (which is directly above the camera) you can get a good quality recording, even in a room full of busy students.
Want to the best way though? Do it at home on the iPad, sitting on your bed with a blanket over your head. Great sound quality, but you do gat a bit hot!