The Air Force was pushing for a deal a few years ago where about 10 C-17's would be leased a private company. The Air Force would have guaranteed a certain amount of Gov't business to whoever agreed to operate the aircraft. They wanted this to happen so they could up their yearly purchase of the C-17 and get a much better deal, saving millions (kind of a volume discount). They also would have effectively had more aircraft available. The C-17s were supposed to have all the wiring for self protection equipment and plumbing for air to air refueling, but it would not have been functional. This was for standardization, lower cost, and the hope that the Air Force would eventually buy the aircraft. So far, nothing has happened and I have not heard of any new developments.
The only problem with operating military cargo aircraft is they are less economical to operate than civilian freighters. They are alot heavier than civilian aircraft due to their 'heavy duty' construction. They also have stuff like a special system that injects inert gas into the fuel tanks as fuel is burned off, to help prevent explosions if hit by ground fire, etc. This system alone weighs over a ton. With a glut of ex Soviet military freighters to serve under developed regions, there probably isn't much of a need for the C-17 in civilian service.