Personally, I think that a withered deathflower would not make sense at all, and is therefore not possible ingame. Because a sim need to keep a deathflower in his or her inventory for incase he will die ones, waiting for a moment like could take a enormous amount of time. Time in which every living plant in The Sims 3 has died already.
Therefore, I believe a deathflower is dead already, and therefore can't withered.
Why you can harvest death flowers by growing death death flowers will still remain a secret if my text above is right.