I have a friend that is a chemistry major and according to them it's easy to synthesize nitrate to nitrite. BUT they're a chemistry major. I have no idea how possible it is for the average person.
"Industrial production of sodium nitrite follows one of two processes, the reduction of nitrate salts, or the oxidation of lower nitrogen oxides. One method uses molten sodium nitrate as the salt, and lead which is oxidized, while a more modern method uses scrap iron filings to reduce the nitrate."