is it possible to add a lead shell to an atomic bomb to make it an anti-matter bomb?

The atomic bombs that are in existence today are either fission bombs or fusion bombs (sometimes called hydrogen bombs, because they fuse hydrogen atoms). The fission bomb will start a chain fission reaction (this is the splitting of atoms such as uranium or plutonium) when the atom splits, it forms 2 or more new atoms (different elements, as they have different number of protons). Plus the energy that was binding the protons/neutrons together is released, plus some stray neutrons go shooting off to split some more atoms, this is the chain reaction. For fusion, you get 2 Hydrogen atoms to fuse to make Helium, and in the process energy is released. This is the same type of reaction which happens in the Sun.

For antimatter, you'd have to collect the antimatter and keep it isolated from matter like with a magnetic field or something like that, until it was time to detonate, then I guess you just turn off the power and let it touch regular matter (like the container). So with antimatter, supposedly it will combine with matter and both will completely be converted to energy, so this would require a lot less mass of material than the other types.