Yes, you can back feed a breaker in the loadcenter if the instructions on the loadcenter say you can. You are required to secure the back fed breaker so you must purchase a hold down kit that fits the breaker and loadcenter. The hold down kits often will only fit in the one designated position in the loadcenter. If there are provisions to replace the main lugs with a main breaker that is usually the better way to go. Many Square D and GE loadcenters have main breaker kits to replace the main lugs.