Several good options already mentioned. About the easiest is adding in shot.
Either chilled or magnum will work depending on what you have readily available. Best course when using it in however, is to smelt it up separately adding in a little at a time. Make sure you have a spoon that you can mash it with as well, as some of the time it will develop somewhat of a skin which is harder and the lead inside of it can't get out.
I used it quite a bit to make up fishing weights years ago and found that depending on which brand, hardness, or even age, some would readily melt and some had to be mashed. What you DON'T want is to have most of it melt and some of it stay in pellet form. That will make things miserable. Just be sure to mash as you go and if it is floating mash it against the sides of your pot. Again only do small batches at first to see how it goes before simply dumping in 5# or so.
After smelting some of the shot into ingots, only blend up a small batch at a time with your WW's and weigh out exactly what your mixing so you can replicate it the next time. Personally I wouldn't make it up in batches over about 5#. Also remember that it doesn't quite do a linear alloy either, meaning adding say 1" of shot alloy into 5# of WW's isn't going to be as hard as adding in 2# of shot into 10# of WW's. The arsenic will do more as it is added in and you might end up with harder than you actually want.
Also you might want to check out the latest Alloy Calculator over on Castboolits in the Lead and Alloy section. The older version I have has several different shot type listed on it, and you can somewhat calculate what you have, and what you will end up with, within a very usable degree of accuracy.
Hope this helps.