Is there a diagnostic term for a person who impulsively keeps secrets? Almost like an addiction, similar to the way a pyromaniac feels about fire or burning...  I don't know if there's even a word for this, seeing as it's not a common experience, but if there is, it would probably end with "maniac"?
Wikipedia has a list of impulse control disorders described in the DSM.

As can be seen from the list, many "official" names of impulsive disorders do not end in mania - eg, pathological gambling, sexual compulsion, compulsive buying, and the one most closely related to what is described in the question: Pathological lying.  And if it doesn't fit in the latter category, then:


  Disorders characterized by impulsivity that were not categorized
  elsewhere in the DSM-IV-TR were also included in the category
  "Impulse-control disorders not elsewhere classified".