In order get plumbers lead to the same hardness as WW's you will have to add all the trace elements found in Wheel weights. That is what give this type lead the ability to be heat treated to adjust the required hardness for your type of shooting.
It would probably be less work to find a tire shop that gives the wheel weights away and start from there and leave the plumbers lead for when you decide to get in to black powder shooting.
Good Luck,
HBL