How to copy a polygon in eagle from one layer to multiple others in library (package) editor?
It's not clear what you are really asking about, but it seems you have a polygon in one layer, and you want to copy it to another layer.  At least that's what I'll answer.

As far as I know, there is no direct way to copy to another layer.  However, this can be done in a few relatively simple steps:

Copy the polygon to some unused space in the same layer.

Group the copy 

 In the command line textbox, issue the command CHANGE LAYER [layer number] 

 Right click beside the group you created from the copy and accept "change group"

 click on the MOVE arrowhead icon

 right click near the group you created and issue the command "move group"

Do a MOVE on the group to position the copy, now in the new layer, to where to want it.