Combine first 3 ingredients in a small bowl; set aside.
Coat one side of each tortilla with cooking spray; sprinkle coated sides evenly with 1 tablespoon sugar mixture.
Combine banana, lemon juice, and remaining sugar mixture; toss gently. Spoon banana mixture evenly onto plain sides of tortillas, arranging just off center of each. Sprinkle each with 1 tablespoon walnuts.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place tortillas on rack; grill, uncovered, 30 seconds or until bottoms of tortillas are golden. Fold each tortilla in half; cook 30 seconds or until thoroughly heated.
Spoon 1/4 cup nonfat frozen yogurt onto each quesadilla; drizzle caramel topping evenly over quesadillas.