Complex sentence: Conversion to the KC-10 involved only minor modifications to the DC-10-30CF, the largest of which was the addition of a boom control station in the rear of the fuselage and extra fuel tanks under the main deck.
Split into simpler sentences: Conversion to the KC-10 involved only modifications to the DC-10-30CF. The major changes were the addition of a boom control station in the rear of the fuselage and extra fuel tanks below the main deck.