guido69 There is no reason that you should not be able to smoke your bratwursts as long as you add the correct amount of sure cure, which is 1 oz to every 25 lb of meat. Since you are smoking them your process is going to have to change a little. You are going to need to get some protein extraction during the mixing stage or you run the risk of fatting out during the slow smoking process. Fatting out is when the fat separates from the meat and renders out of your sausage leaving you with a dry less tasty sausage. Fatting out happens when you are cooking your product at low temps for a long time like you do when smoking. So, make sure you get enough protein extraction and consider adding a binder like carrot fiber or sure gel.
For more information on this check out this video https://www.youtube.com/watch?v=wStH-RtQUY8
Let us know how they turn out.