I will answer your question in terms of theory, presuming that demonic possession is a real, if rare, thing. Yes it is possible for an atheist to become possessed, in fact there are several examples of this happening to atheists, members of other religions or people completely removed from religion altogether. A devil would most likely try to create the largest amount of chaos possible and if that means using the body of an atheist, then they will.
Can they be exorcised? Absolutely, it helps if the person possessed tried to expel the demon themselves, but it is not necessary. They can certainly be helped regardless of their beliefs.