I see two ways of doing this. If you work within Keynote directly, you can import an audio file from iTunes on each slide. It would be better if it had an option to record on the fly from a microphone, but no such luck. Once you've done that, you can then export the whole thing to Quicktime. The only complication is that the Quicktime option looks to only allow user controlled transitions, which probably would require a .mov file, not an mp4 file, or specify a standard time per slide, which might cut off your audio.

To do this from iMovie, you'd be best off exporting each slide individually as an image (Keynote also has an option for this, you don't have to do them one by one), and then import each image as a separate clip in iMovie. You can then record your audio track, and then adjust the time for each still image to sync up appropriately with the audio. You'd lose the slick transitions, but it would get the job done.