Timeline Entrances - Copy/Paste

There is a way to do this. 

Select all the elements that are part of the animation and copy them as usual. 
Go to your new layer/slide and right click on the canvas. 
Choose Paste Special... (or just press ctrl-alt-Z)
Choose all the features you want to keep from the original (duration, start time, animations, triggers) and press OK.

It will not capture Slide Triggers, but will capture Object Triggers. So, if you have an object enter scene at 32.5 seconds in the original, then it will do that in the copy as well. If you have a trigger that emphasizes one of your objects when another enters the scene, that also works. 

If you have a trigger that changes state when the timeline ends on the slide, it won't copy that.

Sample 1:

Select the objects, paste them onto a new layer using Paste Special.

original:

Result:
Object triggers and timeline copied successfully.
Slide Triggers missing.

Sample 2

Copy the objects like above, then select & copy just the Timeline triggers and paste those (standard paste, special not needed).

Original:

Result:

It kept the triggers and general details, but needs to have the objects reassigned.