how to import slides into iMovie

I've got a PowerPoint presentation (which I will import into Keynote for the purpose of easier transfer). What I want is to bring it over to iMovie, but I want to manually set the duration each slide is shown for. I'll be adding my own sound track (it's a presentation) so each slide is to stay up for a different amount of time. So, I can't simply export the presentation as a QT movie with a fixed duration for each slide. What's the easiest way to export my presentation into iMovie so as to be able to add sound to each slide and use the length of the audio to tell it how long to leave the slide up for? Then I'll export the whole thing as a QT movie.

thanks!

Mike