Copied answer from other comment - Editing the normals isn’t possible in the editor, but it is possible to output a different normal value in the material to achieve the same result. This would work since the OP has only one directional light.
You can set the normals in a material by plugging any vector into the normals input. You will want to turn off “Tangent Space Normal” in the material, and then set a vector input parameter that says the location of the light source. Get the vector from the light source to the world position, then normalise this and you’ve got your new normal value.