It's possible to not only add simple accessories attached to specific bones, but we can also create new skinned models that adhere to the NPCs. An example of this is clothing. We'll make something simple here to give you an idea.
I've made a simple skirt here.
Add Skin Wrap, and use the reference model BodyNaked_LP as its parameter model. We'll need to converto to Skin to manually fix the bone weights.
Let's do a bit of planning. Since this is an above the knee skirt, we'll want the thigh and pelvis bones of affect the deformation. Other bones might not be as necessary. We'll focus on those.
You can enable Paint Weights. Configure your brush in the "..." button if you want to. Toggling "Paint Blend Weights" is also very helpful sometimes.
Use all of the tools you need to get your skinning as good as possible. After that, export the your accessory and get it to Unity.
Add the skirt to a new or existing mod. Open up the Accessories field, set its Size to 1, and under "Accessory", drop the GameObject that contains the Skinned Mesh Renderer of your model. The "Bone Type" field here will help set the "bounds" of your model. The bounds help determine when an object of off-screen, so it can be culled. Select the closest bone, or the one you feel appropriate.
Note: Accessories and Bone Accessories don't support LODs.
Accessories will be "destroyed" when an NPC is gibbed. If you want your gibs to maintain the accessory, you need to add that same accessory to the gib you want. If you are adding an accessory to a vanilla NPC, make use of the generic gibs supplied.
Note: Right now, options are very limited for adding accessories like these to vanilla gibs. I may work on this aspect in a future update.