Such salts are usually mixtures - my take is that iodized salt is just NaCl plus some NaI or KI added. In general you can't write a balanced equation for mixture, usually you can only write balanced equation for every individual compound of the mixture (assuming they react at all).
Sometimes chemistry of the mixture gets more complicated and more interesting, but I doubt that's the case.
Don't they have to put composition information on the box? No idea how it looks US, I think in EU it is obligatory for every food product.