Hello Bilas,
This can be done through the Circular profile. You can add multiple layers and control the each layers geometry (refractive index distribution). Once you have the refractive index profile you want you assign this profile to your lens. For example here is an example of a parabolic graded index ranging from 3.48 to 1 over 2 microns and the results of it assigned to a lens with a radius of 12 um.