Similar to player corking in that lead is put into the bat barrel via drilling a cylindrical hole in the top of the barrel.
But I have never heard of a player wanting to artificially - increase - the weight of his bat by lead, or other means.
It has been rumored in the hobby that individuals have added nails, lead pellets, lead cylinders, etc. to bats to increase weight so that such bats might better correspond to player ordering records.
So if you have a bat of Babe Ruths and its length is what Ruth ordered, but its weight is 5-10 ounces lighter, and therefore a reviewer might call the bat a pro stock and not a more definitive Ruth gamer, someone might put more weight in the bat, to attempt to fool the evaluator.
so if a bat feels unusually top heavy, or looks like it has been drilled, or has an array of small touch ups, you may want to bring the bat to your dentist for x-ray prior to purchase
My dentist also zaps cavities via laser, no drilling, in under a minute, with no pain and no need for anesthetic. Its amazing.
Was typing as sicollector wrote - interesting, had never heard of a leaded warm up bat before.