Adding shotgun pellets to pure lead won't do much to harden that pure lead. Shot is made in 2 main configurations. Regular and magnum. The regular is commonly called "chilled shot". That means the pellets were dropped into very cold water to "chill" them quickly. There's only about 2% antimony and a trace,(less than 1% tin), which allows a hard skin on the pellets. It's what us casters call water drop hardening.
Magnum shot is about 6% antimony, with enough tin to keep the antimony in solution with the lead, about 1.5%. Magnum shot will harden pure lead, but you'd need a lot of shot to do it. Since a 25# bag of magnum shot is right around $50.00, it's a VERY expensive lead to use to alloy pure lead.
Roto metals sells a lead additive that makes for very good alloy by simply adding it to pure lead. It's 30% antimony to 70% pure lead.
http://www.rotometals.com/product-p/30_antimony_70_lead.htm
You
WILL need to add some tin with it to keep the antimony in the alloy. Tin is required in any alloy to keep antimony in solution. If you have too much antimony without tin, you will see antimony crystals surrounded by pure lead. Go to that link at Roto metals for the super hard alloy, then click on the link to the LASC about how to use it to arrive at a useable alloy for your needs, or go straight here;
http://www.lasc.us/SuperHard.htm