I think "왜 이렇게 단호해요?"mean is "Why do so decisively?" and "단호박이세요?"mean is "Are you pumpkin?" So,"단호박이세요?" is kind of strange but of course you can say.

단호박 is a kind of a pumpkin ,which is more sweet than an ordinary. the 단 of 단호박 means " sweet"