Adding some weight to the handle, if you keep the weight as close to the butt cap as possible, will have a minmal effect on swingweight, maybe a point or two increase to the swingweight at most. You cannot reduce swigweight, unless you remove material from the upper part of the racquet, make the racquet shorter, or go from a thick gauge string to a very thin gauge. Added weight to the handle can potentially make the racquet feel a bit more manageable, due to the increase in a head light balance, but it will not lower the swingweight.
Inertia is a product of mass times distance squared. Swing weight on a tennis racket is calculated from a point 10 up from the butt of the racket. Any weight you add to a racket increases the SW by mass in Kg times the distance in cm squared from the 10 cm axis. Adding 5 g to the butt of a racket increases the SW by .005 x 10*10 or 0.5 points. The point on a racket where addition of weight has the least affect on SW is the 10 cm axis not the butt. Adding mass at the butt has the same affect as adding mass 20 cm up from the butt. The point where adding mass has the greatest affect on SW is the point farther from the 10 cm axis or the tip of the racket.