Joining Pellet Vent in a Wall Thimble

Is there any problem with joining two pieces of vent pipe inside a wall thimble? Seems like I read something somewhere that this was not code, but my stove manual doesn't say anything about it one way or the other.

It is fine
Good idea to make sure it is sealed good and you use 3 screws at the joint (all your joints) to secure it.

Excellent. Thank you very much for the reminder to use screws, too. I usually remember such things after it's too late and have to pull it apart again.

That's about the only good way to transition from black pipe to galvanized or SS. If you do use screws on the joints make sure they don't penetrate the inner wall of the pipe.