Yes, you can add the new insulation on top of old insulation, unless it is wet, rotted, or infested. 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 there are sign that ants, termites, mice, squirrels, or other pests are living in the attic insulation, remove the infestation and treat the area (including closing access holes) before adding more insulation.
If you discover vermiculite insulation in your attic, be sure to have it tested for asbestos 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. Learn more at: https://www.epa.gov/asbestos.
Remember, there may be Home Performance with ENERGY STAR contractors in your area you can hire to help 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.