Stephen Dodd Hughes discusses this in his book "Custom Shotguns." He as a method for doing it (which does NOT include pouring melted lead into the hole in the stock, but is rather fitting a piece of molded lead into the recepticle in the stock/forearm, etc...

Why not just drop in a mercury recoil reducer? They're available in practically any weight, and it would be a lot easier to both install and remove than just plain old lead.

Good Luck