Are ADHD and narcolepsy related? In what sense?
If you mean, are the two conditions related because they are both brain-based conditions that affect alertness and sleep? Yes, they are related.
If you mean, are people who have ADHD and those have narcolepsy sometimes helped by the same medication (modafinil, or Provigil), the answer is yes.
in our current medical system of tall and narrow "stovepipes," it seems the diagnosis you will receive often depends on the specialist you consult. Many people with untreated ADHD suffer sleep deficits of various natures; this could lead some narcolepsy specialists to diagnose these patients with narcolepsy and entirely miss the ADHD.
I hope this helps to answer your question.