France regards its nuclear mission as sacrosanct and a national obligation. They will be very unlikely to 'share' anyone else's tankers to support this mission.
MCCE can provide ATARES to non-tanker nations under certain circumstances, but I cannot imagine France being prepared to rely upon such a service for their nuclear mission.
However, an update programme for the C-135FR in particular and perhaps the KC-135R would be an entirely plausible and pragmatic way of delaying the need for 'Tanker de Future'.