Using one of this solutions will be more efficient and saves you time, because there's no more need to manually sync the presentation and the voiceover.
Free solution:
- Keynote can record a running presentation with the mic-audio.
- QuickTime X has also a screenrecord-feature incl. audio.
Lowcost solution:
- Specialized screenrecording software like ScreenFlow, Camtasia, Screenflick, Screenium, etc.
Pro solution:
- Capture hardware like Ninja Atomos, HyperDeck Shuttle, Intensity Pro, Aja IO, etc. can record directly with ProRes-Codec.
A professional solution with capture hardware can record directly in Final Cut Pro X (and other NLE) native codecs/formats like ProRes422. This saves much time. In addition you'll get a recording with smtpe-conform timecode. – There are even more advantages, there will be more important if you record live presentations.