Yes, you can use a pellet grill/smoker in the rain. However, if you are going to open the pellet hopper to fill it up with pellets you need to provide suitable cover.

If rain gets into the pellet hopper it will expand and destroy the pellets. If a lot of water gets into the hopper, the pellets will expand so much that they will expand within the auger tube and block it up, which can take quite a while to clean out.