Bring chicken broth and water to a boil in a saucepan; remove from heat and stir in couscous. Cover saucepan and let couscous stand until the liquid is absorbed and the couscous is tender, 5 to 10 minutes. Fluff with a fork.
Place couscous, cucumber, green onions, carrot, and parsley into a large salad bowl. Combine olive oil and lemon juice in a small bowl and pour over the couscous and vegetables; stir cumin, salt, and black pepper into the tabbouleh. Mix in feta cheese just before serving.