If the aliens are animals, they would have to meet the same requirements as regular earthly animals, and we would also need a tradition that they were kosher (this would probably require a navi, so don’t hold your breath). If an alien were sentient, the question (addressed in science fiction) is whether they could become a ger. If a plant, it would probably be kosher – but consider what if the plant were sentient and wanted to convert. The probability is that any alien food source would be toxic for humans, or at least would be inedible due to a different chemical composition.
