Long-lead time for some of the components (e.g. big forgings) is in excess of 24 months. Last C-17 is scheduled to roll out in less than 9 months.
Now, I have no visibility of what arrangements Boeing may have made with the various suppliers, but keeping a line open for over 12 months, essentially doing nothing, while new long-lead parts were sourced would be horrendously expensive.
So it may be possible for Boeing to extend the C-17 production, but it would be very expensive and the resultant airplanes would have to sell for a significant premium to justify the expense.
All a long winded way of saying, it's highly unlikely there will be any more C-17s built beyond what's currently planned.