^ integral non-dualist, very interesting. I would call myself integral and advaita vedanta [which in english basically means/translates as nondualism]
Anyways, sure they can! I view Ken Wilber as atheist [and enlightened] and he uses the word spirit quite frequently in his writings. I think a lot of the answer depends on what one means by theist. I think many people try to avoid it, but an extremely strong case can be made that Buddhism is atheistic. By extension, other religions that hold the same ultimate viewpoint would be as well.
I think of theism as " an external ultimate independent creator god stood outside of creation and made creation, and said creater god is not us/ we are not said creator god." Under such definition Buddhism is absolutely atheistic, as would be Hinduism as well, for example. I think we could define theism in such a way that Buddhism, and others like it, then became theistic. So, like I said earlier, a lot of the answer comes down to "what does theism mean?"
Personally, I've viewed myself as atheist polytheist before, but then I decided there were better words available and that it didn't matter.