Custom Vertex Weights/Normals come with the engine to provide support for mid-poly to low-poly models to use weighted normals that were done in external 3D software.
Source 2 accepts custom normals, but it is hidden away at the moment and is checked off.
We need to add RenderMeshMarkup to the mesh you want to apply it to:
Then we need to check "Use Expensive Tangents":
Here is a before and after: