How to use
Requirement is Unity 2019.4.31

The addon adds menu entries to the GameObject menu and Tools menu on top. The GameObject context menu is also accessible on objects in a scene. An Accessory Installer menu entry is also accessible for Project assets.

Accessory Installer

Avatar: Your avatar.
Avatar Mesh: The SkinnedMeshRenderer for your avatar model. Is correctly detected automatically in the most cases.
Accessory: The accessory you want to add. Can be an object in the scene or also a prefab from the assets.
Accessory Mesh: The SkinnedMeshRenderer for the accessory model. Gets also detected automatically.
Method: Select if you want to keep the Hierarchy clean by using ParentConstraints or if you want to use the "classic" way of moving bones to the avatar's armature.
Prefix: For the HierarchyChange method. When moving the armature, this prefix is added to the bones of the accessory.
Mapping: Define which bone of the accessory belongs to which bone of the avatar. Click the X if you don't want to map certain bones.
BlendShape Sync

Avatar: Your avatar.
Avatar Mesh: The SkinnedMeshRenderer for your avatar model.
Accessory: The accessory object.
Accessory Mesh: The SkinnedMeshRenderer for your accessory. This should contain the BlendShapes you want to sync.
Sync BlendShape Values: Write the current values of the Avatar's BlendShape to the accesory.
Mapping: Select which BlendShape of the accessory is related to a BlendShape of the avatar. Click the X if you don't want to map certain BlendShapes.
Sync Animations: Search in all used animations for animated BlendShapes. If an animation contains a BlendShape of the avatar that was used in the mapping above, it adds the Accessory BlendShape to that animation using the same keyframes.
Animations: Gives you an overview of all animations that animate BlendShapes of the avatar. Use the checkbox to enable or disable the syncing of these animations.