I really dont think its possible as if car isnt design in such way then where you will accommodate rear differential, 4x4, and drive shafts.
For that you need to alter the rear axle completely and make space in center for adding 4x4 gear. To my knowledge even if some how you can do this it will cost more than the cost of buying GS430 4wd.
So sell IS and buy GS, my all time favorite car.