I managed to successfully cast from DamageEvent to PointDamageEvent in code. I used a C-style cast, rather than the UE4 Cast<> function, but it worked out:
const FPointDamageEvent* pointDamageEvent = (const FPointDamageEvent*)&DamageEvent;
That will give you the pointer pointDamageEvent to use, and it worked for me when I tested it.