Hi Elang,

in ZBrush you can follow this steps
http://docs.pixologic.com/user-guide/3d-modeling/sculpting/bas-relief/

Sometimes is it hard to get a clear and clean mesh from a 3d package to ZBrush. So make sure the mesh ist a solid, the normals showing in the right direction

Another solution can be:
Export this model as obj and import it to Cinema 4d and use the Volume Builder and the Volume mesher to generate a volume.
This solution is more flexible, but it can be need a lot of ressources of your hardware. Try first with a bigger voxel size!

https://www.youtube.com/watch?v=H-cykUwp4yU