Being a believer in anything isn't part of being an atheist, just as many believers in God believe in aliens even though it's not part of their dogmas. The people I've met who claim to believe in aliens a) also claim to have credible evidence and b) do not have a personal relationship with aliens most of the time. But I should also point out that many scholars of religion have classified UFO beliefs as a kind of religion (e.g., a good book on this is American Cosmic: UFOs, Religion, Technology), and so the UFO-thumping atheist or theist is really just a competing religion like Buddhism...
