This tutorial shows, how to create a mesh from a sprite. The generated mesh can be used with the Skin2D component. The mesh creation is done within Unity and doesn't rely on any external 3D software.
Download: github.com/Banbury/UnitySpritesAndBones
Notice: For this to work the sprite must be readable. In the Project window select the sprite and change 'Texture type' to 'Advanced'. Then check 'Read/Write enabled'.