What do you mean by adding fuel tanks? Are you using some feature I didnīt recognize over the years ? Where do you get the information how much further your aircraft will go instead of an absolute number ?
You can either lease/buy an airplane that is already built that way or change seating. Either way system calculates exactly the range of that plane with actual configuration,available seats and with needed fuel for planned route and regarding the longest possible runway that is used for take-off/landing. What special problem do you have ? Please talk details.