My first impression would be to say No. Knowing the alloy make up of the lead you are casting will give you an indication of what you may be able to get in hardness with the correct heat treating method.
Just alloying is not going to do anything but change the base hardness of the lead itself, meaning the lowest BHN. Heat treating will give you a variety of hardness, depending on temp and time, so that the alloys are allowed to do what they are designed to do.
Same goes for alloying steels. Heat treating and method of heat treating is what governs the hardness of a material with specific alloys.