Yes, you can add the new insulation on top of old insulation, unless it is wet. If it is wet or appears that the insulation has previously been wet, you should look for the cause and repair the problem to prevent a reoccurrence. Remove any wet insulation. Wet insulation can lead to mold, mildew, or even the rotting of your ceiling or roof rafters.
If you discover vermiculite insulation in your attic, be sure to have it tested for before doing work there. If the test reveals that asbestos is present, the vermiculite should be removed by a certified removal expert before disturbing it with the installation of more insulation.
Remember, you can always hire a local home energy contractor to evaluate your attic and make suggestions on how to proceed.
Learn more on the Seal and Insulate with ENERGY STAR web site at: www.energystar.gov/sealandinsulate.