When casting with wheel weights, if you continually just add weights to the pot, throw away the dirt without fluxing and stirring and cast with a spoon or ladle, the non-lead (lighter weight constituents) content of the upper layer of metal in the pot will continually increase along with bullet hardness. Been there, got the tee shirt. Means that the bottom of the pot is of higher lead content.
If you were to melt down a drop bottom pot full of wheel weights and leave it sit and cook for a long time then the bottom would have a lesser percentage of alloying elements. It would be softer. But, who knows how much time would be involved to get what result? And, you still wouldn't have pure lead softness if that's what you're after. But it might be soft enough.