Oh yes, but adding all of those tidbits isn't a particularly difficult (and certainly not a technical) task as a writer. JKR is very good at going back and joining the dots, picking up on little things she's added here and there and ascribing new meaning to them in retrospect, but it's rarely constructed in a setup-and-payoff style where any of the later plot developments are clearly foreshadowed. With story construction I'm referring more to the purpose of her writing choices, the *way* in which she tells the story and the reasons for what happens in it.
The most striking example is probably how she handles character death, it's horribly misused in almost every case to the point where it becomes completely baffling in the final book. Most writers with a technical background will understand that a dramatic device like that has to be used very carefully and appropriately, with the right emotional context in mind and for a very clear purpose within the story. As it stands off the top of my head the only character I can think of that met an appropriate and meaningful end in writing terms was Dobby.