Radioactive materials are dangerous because their subatomic makeup is unstable, meaning they emit dangerous particles as the atom degrades from the inside. So it would seem that, in order to fix that, you'd have to re-populate the innards of the atom with enough protons to make it stable again.

It's worth noting that transmutation is definitely possible. The alchemists of old would be thrilled to know that we can, in fact, make gold from base metals (in a nuclear reactor). The problem with that is that the cost of doing so is worth more than the gold could be sold for.