I'm not sure converting previously owned frames would be more economical than building from new in this instance. If you wanted to make a tanker to sell AAR time to the various militaries of the world currently enjoying a 'capability holiday', a la Omega, then converting retired airliners does work. But to meet the RFP there might be too many alterations required. E.g. NVG compatible flight deck (not sure how the 787 screens will do there...), 1200 USG/Min offload via the boom etc.
Ulrick has the right idea until the KC-X fiasco is sorted-out, but when it is I expect new build aircraft will be part of the winning proposal.