Can I add more diet terms?

The default "Diet" taxonomy is a special one that's directly related to the schema.org suitableForDiet metadata. This is a fixed list of diets that can be found here.

Since these are the only diets that are allowed for this metadata field, it's not possible to add others. You should also not be renaming diets that you don't expect to use to something else, as then you'd be incorrectly tagging these recipes as being a specific diet in the metadata.

If you need more flexibility and don't care about having this in the recipe metadata, creating your own Diet recipe taxonomy is an option!