Mush layer can be used to copy an area of the model from a library on top of the animation. Before you create a Mush layer, open the Shape Browser and select the model you want to copy from, create the layer and use the mask brush to paint the area you want to affect. Instead of use a model from a library you can also use another frame from the animation, just go to the frame you want to copy from and save a morph target, go to the frame where you want to copy to and create the mush layer and use the mask brush to paint the area it will affect.