I think you will find a production slot has already been allocated.
It is my understanding this matter was being discussed in Long Beach 12 months ago.
There was a Long Beach sourced story in Aviation Week mid last year talking about the 5th RAAF C-17 and a separate joint purchase with the K1W1's.
I would think the cost of the Christchurch earthquake has probably scuppered that deal although RAAF C-17s have been very busy flying people and material across the Tasman.