Hey Art
I did my C208 rating in Botswana back in 2011, it was a 3 day ground course and then 5hrs flight training.
I then had it placed on my CASA license without to much hassle, and it shows as a separate rating.
This was under the old license system in Australia, but I am fairly confident that it's still a seperate rating under part 61.
I understand what your trying to do, and recently went through a similar problem which required me to get a SA license.
I just don't believe that CASA will allow the placement of a rating directly onto the Oz license by an outside training provider. But I am sure they have exceptions.
Obviously fisrt point of call is CASA, and I wish you the best.
Let me know what they say, curious myself.