Can one be an atheist and believe in an afterlife?
Of course. In fact, many do. Moreover, as I understand it, mono- and polytheists didn't invent the concept of the hereafter. Atheists did. Though there's some argument on that point. Many claim that the concept was invented by polytheists, but all we really have to go on is crude paintings on rocks and caves, so it's far from certain.
In any case, the lines are kinda fuzzy. Hindus are technically poly, but many of them shove it all up to Sree Rama as an ultimate deity, so they are rather mono, as polytheists go. Similarly, Roman Catholics are technically mono, but have you ever witnessed Extreme Unction? It's all like, "Saint Francis and Saint Benedict, pray for him. Saint Mary and Saint Joseph, pray for him." Even in a run-of-the-mill penance, one is expected to appeal to the Holy Mother. And of course there's the Christ: Three persons, one God, is the official line, but it's rather a subtle and tenuous claim on exact monotheism, at least compared the Muslim, Protestant, and Jewish varieties. And of course, the Buddha never postulated the existence of any god or gods--and thus Buddhism is often classified as an a- rather than a poly- or monotheistic religion--yet most Buddhists have a thorough grounding in the school of re-incarnation. Then again, many Buddhists, especially in the Far East, have a whole host of gods, starting, usually, with a city god or village god, so it's probably not actually good form to classify the group of religions known collectively as Buddhism as atheistic. Given your username, which hints at that flavor of Buddhism that advocates direct enlightenment through meditation, I expect you probably should already know this.
But you don't even have to look there to find atheistic versions of afterlives. The Dao, as well as most strains of animism and ancestor worship, are all decidedly atheistic religions, yet all of them have a profound conception of the hereafter. And there are
plenty of regular, garden-variety, trailer-trash, white Americans, the kind who'd probably be Libertarian if they ever bothered to register to vote, who have absolutely no taste for organized religion, but who are quick to make mention of the ghosts that have invaded their homes, and quite certain of Mama's presence during their latest drunken encounter with the oiuja board.
short attention span theater version: I don't think you should conflate metaphysics with religion. We can examine one independently of the other. Moreover, even adherents of proper atheistic religions often have some ultimate reality involving life after carnal death. I've also known of irreligious sorts of people who, from time to time, venture to dabble in questions of metaphysics, completely devoid of religious observation.
