As far as I can tell Karamba3D does not yet support orthotropic materials (or any kind of anisotropy), nor does it support parametric materials.
Yet if you stick to isotropic materials, your can discretize your material property variation : you can split the shell in smaller patches (or “elements”) to which you assign different properties. Ideally, if you reduce the size of the elements towards zero, your model will tend to a continuous gradient of structural properties. If you are just looking at global behaviour and not concerned about local values or max/min values this approach should be enough. Don’t trust any local value though as the discontinuities of material properties and the slicing of the mesh will introduce singularities in the FEA analysis.
If Orthotropy is an important concern, you can set up a makeshift solution by creating “stripes” with different material properties, which will give you a very rough first approximation of the global behaviour (again, nothing local).
I’ll recommand going directly to more suitable software for your problem, unless you specifically need the features of grasshopper.