Pre-compose? I'm not sure what exactly you are describing. Other than that you can add a keyframe at any point by clicking on the little rhomb right between the next/ prev key arrow that appears at each property once it's animated. This will use the current values.
Let's say I have a camera layer. its duration is 10 sec. Next I increase the total composition duration to 15 sec. After that I need to "insert" 5 sec freeze segment into a camera layer. The 1st and the last frames of that 5 sec segment should have the same property to freeze camera. how can i do this?
also, the camera track has keyframes for all frames.
So I need to insert "empty" frames inside with no keys
I think what you're looking for is hold keyframes. Check out the help files. I'd give you more if I were not answering from my iPhone.