Hi
Although you can "freeze" the action easily enough, this won't always look quite right. Although you say that you don't want to alter the synch of your associated audio, any method that alters the length of your video will have some effect on the sound, and "freezing" the action certainly will unless you are prepared to play about with the sound track.
If you find you cannot shorten your narration to fit the video clip then you should try slowing down the action in the clip section concerned as I am going to describe. The first thing to do is save the project then you can always reload it if things don't go OK.
I am assuming here that you have recorded your bit of narration as a separate clip, or have cut it out of a complete narration and it's in the media list.
Now you will need to cut out (not remove...just isolate 
) the section of your video against which you wish to place this narration.
Play your video to the point where your piece of narration is going to be placed. It's better to stop at the point where the shot or section or scene in question changes, (and provided you don't have the previous bit of narration there.) Adjust the red cursor line to exactly this spot by stepping forwards or backwards with the < > buttons whilst monitoring the right hand preview screen. Zooming in can help here. Now split the video by clicking the button marked "Split" on the left of the screen.
Play on to the end of the shot and split the video once more.You will now have isolated the clip against which you want to place your narration.
Now drag and drop your narration clip from the media list to soundtrack 1 so it is under the isolated clip. If you are correct in your post, the narration will extend beyond the end of your isolated clip.
Left click the isolated video clip to select it and then left click the effects tab. Select the "Speed" effect. Slide the slider to the left and you will see your isolated video clip increase in length. You can now adjust its length with the slider until your narration fits. You can, of course then drag your narration clip left or right so it starts and finishes where you want it to within the elongated video clip. Note that the frequency of the audio track will alter (gets lower) but provided the speed ajustment is not too much and the soundtrack is not music,you may not notice too much difference. This is one reason why you should split the original main clip where the shot starts and finishes as it is at these points that the audio track would normally change anyway. The synchronisation of the audio track will not alter using this method.
Hope this helps
Nat