This is actually may be trouble. There is several ways to do that:
1. Keep reference in metadata to original item id to obtain it's food points and icon, so it will acts completely as original items. It is not hard for food that is not metadata sensitive, and impossible for items that hosts multiple items on a single item id - I know that there is addons that adds new food.
2. Same thing, but keep reference in nbt, it will work for any items, but make items non stackable.
3. Also I may try to use enchantment system on original items. It will not need additional items and save effect even after addon uninstall, but it also will make items non stackable.
4. Make an item that will act as uniform irradiated food, e.g. it will be simply rotten flash with another static icon. Least cool, but best working.
Please don't. It would make the mod useless. Also, in real life, irradiated food is'nt radioactive, unless it has been subjected to REALLY high levels of radioactivity for long periods of time.