I know this is an old thread but this is something I do ALL THE TIME with my Traeger and with excellent results. Sure, starting with fresh chicken and going forward is 5 times easier, takes less time, and tastes better. But using your Traeger on those frozen breasts from Costco is the ONLY way to fly!
I modified the usual as follows:
Rub frozen with favorite rub. Has a hard time sticking, so save some to apply during cook.
Traeger set at 350.
This part is somewhat hands-on compared to traditional:
I cook at 350, flipping frequently, for roughly 15 a side until chicken is thawed through...you can tell when you flip it. This feels more like traditional grilling but must be done evenly.
Once thawed, I go with "Basic Traeger Chicken Breast" method which is 30 minutes total cooking time, turning once. I've done this at 350 for the full 30, and I've also cranked the hit to max (400-415) and cooked about 12 on each side.
I just did this today and with great results...remember, you are starting with what I feel is probably one of the plainest tasting pieces of cheap chicken breast you can get and I STILL get tender and juicy chicken that tastes good.
It's always better on the Traeger - period.
And here is the end-result...hopefully this photo does it justice:
This is one cut in half with the juicy factor clearly visible. Again, if you have a freezer bag full of these, this is the only way to cook and eat these things!