You cannot change the existing Exception classes.
But you can catch the exception and convert it to a CustomException:
try
{
try
{
// Do you thing.
}
catch(Exception e)
{
throw new CustomException("I catched this: " + e.Message, e);
}
}
catch(CustomException e)
{
// Do your exception handling here.
}
I don't know it this is what you want, but I think this is the closest you can do.