The materials that are used in FDTD analysis MUST abide by the Kramers-Kronig relation. It is possible to fit the experimental data to a Lorentz drude model and then define a material with those coefficients. You can Matlab code from quick searches that will fit these problems.
http://fdtd.kintechlab.com/en/fitting
An example of a Matlab program can be found at the link.
Regards