An easy way would be to export each powerpoint slide as a JPEG file. Import these into an iPhoto album. Drag the iPhoto album into your movie through the Photo Browser and add your voiceover.
It might also be possible to export your powerpoint presentation as a QuickTime movie, but it would be important to do it as a timed presentation rather than a manually advanced presentation, because iMovie will not import manual advance tracks, closed captioning tracks, chapter tracks, etc.
I have imported presentations from KeyNote but have never tried a PPT.