I'm not sure if you want to replicate frames that have content in them or you just want to extend the timeline. If you just need to extend the timeline then you can just grab any existing frame in any layer and "pull" it out to the right until you get to the length that you want.
One way to do this is to create a new empty layer at the top of the timeline. It will be as long as the current length of the movie. Grab the last frame and drag it out to the right. When you reach that frame, click and drag down through the rest of the existing frames in the timeline and hit F5. This will extend the rest of the layers to that new length.
Thanks, Rob. I know about that method of extending the timeline.
I learned through another generous person on this forum that my animation works better when it is near the top end of the frame limit. So I am adjusting hundreds of files and retaining all the characteristics of the animation. So that amounts to F5 X 4 on every frame. Not so bad for a single file, but with the number I have to modify, it is taking a lot of time.