Volume rendering is equivalent to sculpting from colored semi-transparent material. Surface rendering is equivalent to painting on a semi-transparent surface. There is no conversion between them. Volume rendering does not even use a surface mesh while fbx file only stores a colored surface mesh. No matter how you process your data, surface rendering will not be able to reproduce the same look as volume rendering (except the special case when you only have 100% opaque objects with hard edges).
If you want to see volume rendering in Unity then you have to implement a volume renderer or find a volume renderer implementation for Unity (there are some, I don’t know how usable they are). If you don’t want to deal with volume rendering in Unity then you have to create a surface mesh by segmenting your image.