Yes, you can safely cook a frozen turkey if you take the following precautions.

• Use the right cooking method. You can safely roast a frozen turkey in the oven. Don't smoke, grill or deep-fry a frozen turkey. These methods may leave the inside of the bird only partially cooked, increasing the risk of foodborne illness. Microwaving isn't a safe option either because it cooks a frozen bird unevenly.
• Increase the cooking time. To determine the approximate cooking time for a frozen turkey, follow this guideline: Take the recommended cooking time for a thawed turkey (usually on the package) and multiply it by 1.5. For example, if a thawed turkey needs to cook for 5 hours then a frozen one needs to cook for 7 1/2 hours.
• Use a thermometer. Remember that roasting times are approximate, so the best way to know a turkey is fully cooked is to check the internal temperature. The entire turkey — including the stuffing — must reach an internal temperature of 165 F (74 C).
• Check the giblets. A whole turkey usually has a package with the giblets and neck tucked inside. If the giblets are wrapped in paper, there is no safety concern if they cook inside the bird. If the giblets are wrapped in plastic, however, they need to be removed. It's difficult to remove a giblet package from a fully frozen turkey. So wait until the turkey has sufficiently defrosted during cooking and then use tongs or forks to extract the package.