In a large saucepan cook pasta according to package directions. Drain. Rinse with cold water. Drain again. In a large bowl combine cooked pasta, ham, zucchini, and, if desired, green onion.
For dressing, in a small bowl stir together salad dressing, yogurt, and basil. Pour dressing over pasta mixture. Toss lightly to coat. Cover and chill for 2 to 24 hours. Before serving, gently stir tomatoes into pasta mixture. Makes 4 main-dish servings.