Adding weight will increase the mass of the gun. Since the force of the recoil wont change (ie. same shells you've always shot)the increased mass will absorb more of the recoil before it hits your shoulder. Its all basic physics explained by Newton's laws. (sorry for the science lesson) In simpler terms the heavier an object is, the more force required to move it the same distance. If your gun weighs more and the force (recoil) doesnt increase you wont feel as much kick.

Its an old trap shooters trick. They make weighted magazine caps, make weighted plugs to put in the magazine tube, add barrel weights, pour shot in the stock. Their are numerous weighs to add weight. You just have to find a happy medium of what you can stand to carry around verus what your shoulder can take. You could easily add a pound or two, but would you want to walk all day with that gun? Thats a personal descion.