Since all frames always have the same amount of layers, creating a layer on a frame will create one on all other frames too.

Create list

Hitting the add button will reveal a menu where you can choose if you want to insert a duplicate of the current selected layer, or a blank one.

Since you cannot create the bottom layer, all new layers are all transparent.

Duplicates will duplicate the current selected layer of each frame.

All new frames are inserted on the next position of the current selected frame.