Blender – Animation Layers v2.3.4

Edit your animation or mocap data quickly using a standard animation layers UI and workflow directly in your 3D viewport with an extra layers functionality.

Animation Layers is an addon that simplifies the work with the NLA Editor into a standard animation layers workflow. You can quickly create, select and edit each layer, and always seeing the end result while working on any layer. Add or Remove Layers quickly, or control their Influence using NLA’s “Mute, Influence, Solo and its Blending modes” options.

When you are happy with your animation, you can always merge your layers or bake into a new layer.
If you want to keep the same amount of the overall keyframes after baking, then you can also use the smart bake option.