In a large saucepan, heat half of the oil over medium-high heat. Sauté onions for 2 min., then add pumpkin and sauté for 5 more min., add brown sugar and sauté for 1 min., stirring constantly. Set aside in another dish.
In the same saucepan, heat remaining oil over medium-high heat. Add rice, chili powder and cumin, and stir until well coated with oil. Sauté for 1 min., stirring constantly. Add chicken broth and water and bring to a boil. Reduce heat and simmer covered for 20 min. Stir in squash mixture, cover and simmer for 5 min. more. Just before serving, stir in butter and fresh herbs, and salt and pepper to taste.