If you can get every last bit of stuffing in that turkey up to 165°F (75°C), then yes, you can safely steam a stuffed turkey in a roaster oven. The manufacturer’s instructions, however, clearly speak only to an unstuffed turkey, and here’s why: you lose a significant amount of heat from the roaster every time you open the top to check the turkey, increasing the cooking time an unknown amount. How are you ever to get the stuffing to the right temperature without A) checking/losing heat, checking/losing heat (and not being ready for Thanksgiving diner until Friday), or B) not checking until the stuffing is almost certainly done but the breast meat is like shoe leather? The chances of hitting it just right are pretty slim, especially as it will take a long time for the stuffing to cook through in such a large turkey.

If you carefully read the roaster’s cooking-time/temperature instructions, you will also see that they allow you to cook a very large “pre-basted” turkey (up to 22 pounds), but a fresh turkey only up to about 14 pounds, and that they estimate it will take two minutes more per pound for the fresh turkey . Again, this is because they know you are going to want to open the top to baste and fuss over your fresh bird, while the average person will just let the self-basting turkey sit and sit until it is probably done (also, because of the fat in the self-basting turkey, it will taste moister even if it is overdone, so there is more margin for error).