Place grated zucchini in colander. Sprinkle zucchini with salt and let stand 30 minutes to drain. Squeeze zucchini between hands to remove liquid, then squeeze dry in several layers of paper towels.
Combine zucchini, chopped green onions, eggs, flour, chopped dill, parsley, tarragon, and pepper in medium bowl. Mix well. Fold in crumbled feta cheese. (Zucchini mixture can be prepared 3 hours ahead. Cover tightly and refrigerate. Stir to blend before continuing.) Fold chopped walnuts into zucchini mixture.
Preheat oven to 300 degrees F. Place baking sheet in oven. Spray large nonstick skillet with olive oil spray. Heat skillet over medium-high heat. Drop zucchini mixture into skillet by heaping tablespoonfuls. Fry until pancakes are golden brown and cooked through, about 3 minutes per side. Transfer each batch of pancakes to baking sheet in oven to keep warm. Serve pancakes hot.