Yes, you can replace fresh spinach with frozen spinach in this smoothie. Raw frozen spinach: If I know I have more spinach than I can eat before it goes bad I often throw it right into the freezer (without blanching or cooking it), if this is the case for you too, use 2 cups of raw frozen spinach in place of raw spinach. Cooked frozen spinach: If the spinach is cooked use 1 cup of frozen spinach instead of the 3 cups like this recipe asks for, due to the stronger taste. If you have never made a smoothie with spinach in it before I would recommend using fresh spinach as it has a very mild taste.