The gold plated solution would be modular system allowing the extra tanks to be fitted in the holds when a pure tanking mission was required and allowing it to be used as hold space the rest of the time.
Which, of course, is precisely the option available with the Airbus A310MRTT! The 4 x 7200 litre Additional Centre Tanks are removed when not in Tanker configuration and may be replaced by normal LD3 cargo containers.