Can you put insulation over existing insulation?
There are many cases in which it is acceptable to place new insulation over old insulation. The main instance in which you shouldn’t place new insulation over old insulation is when the old insulation is wet or decayed. Also, keep in mind where the vapor barrier is. If there is a layer of vapor barrier between the new and old insulation, it can trap moisture and lead to mold growth.
If you're not experienced in insulation installation, hire an insulation contractor near you to take on this job.