Can croup cause pneumonia?

No: Croup is a symptom of a respiratory infection, which in some, may also cause a lung infection.

Not exactly: Croup describes a clinical syndrome that involves infection ( usually viral) of the upper airway and maybe the air tubes ( bronchitis) so it can not "cause pneumonia" however some of the viruses that can cause croup like symptoms can also infect the lower resp tract and cause pneumonia or infection of the air spaces in the lungs.

Sometimes aspiration: Most pneumonia associated with croup, upper airway obstruction, is related to aspiration during respiratory distress, Sometimes lower airway viral bronchiolitis will occur concomittantly with croup.