Is it possible to add narration to a score in Sibelius 6?  If so, how do I do it?  Thanks!

That depends on what software you have at your disposal.  If you have Kontakt, you can create an instrument in Kontakt that has all of your narration samples mapped within it.  Then you just run Kontakt as a plugin in Sibelius, add a simple pitched staff, and make sure the channel numbers in the Mixer and Kontakt correspond with each other.

If you don't have the full version of Kontakt, you can technically load in an audio track in place of video, but that will probably be more cumbersome than it's worth.