Bring 1 3/4 cups water and basil to a boil in a medium saucepan, and stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork. Combine couscous, ham, green onions, cheese, and corn in a bowl; set aside.
Combine 3 tablespoons water and remaining ingredients; stir well with whisk. Pour dressing over couscous mixture, tossing gently to coat.