Is it possible for humans to conceive of being dead?
Sort of, think of what it's like when you're asleep, not dreaming. Or if you've ever been knocked-out via anesthesia. No-consciousness, just a gap in time.

The question can't really be answered though because it's asking how one might perceive a lack of consciousness, consciously.

Here's an interesting study on the transition from an unconscious to conscious state via the use of anesthetics:

http://www.jneurosci.org/content/32/14/4935.full