Perhaps there is another scenario behind this aircraft, probably cheaper.
After the retirement of B707-320 from New Cromer Airlines (or what ever this airliner is called), the ex-commercial B707 were converted to full KC-135 (Australia and Israel also did this in real life, converting ex-commercial 707s to takners). So, New Cromer B707-320K will have the JT3D from the beginning and later can re-engined them with CFM56.
Strictly speaking, you can´t convert a 707 to a "full" KC-135 because, although aesthetically similar, their structures are different... As Morten812 said, USAF 707s were C-13
7, but I know what are you saying...
The only boom equipped 707s that I know are the Israelis (with a domestically designed boom), the Saudis KE-3 (based on the E-3), and the Iranian ones (perhaps the most similar to yours)... Of these, only the Israelis were "retrofitted with". Australia, like almost all 707 tanker users, only added hose and drogue pods... So yes, It´s possible to modify a 707 with a refuelling boom (or a Tristar like before), but I think is
far simpler to re-engine an ex USAF KC-135A with TF33s (TF33/JT3D were turbofan version the J57/JT3C turbojet) than to mess with structural modifications necessary to fit the boom and related systems into a 707 ex-airliner... And the KC-135s come with a main deck cargo door...