Hi Marlboro,
not flying either of the aircraft but i have done some research on behalf of my boss.
Both aircraft have a range of about 4000nm without extra tanks, with all the tanks fitted about 6000nm, where as you can configure with as many extra tanks as you like, up to alimit of course. As mentioned above it needs certification if you change the configuration.
I was told by pilots who operate the BBJ that anything more than 5500nm is a waste as yo can hardly take any passengers and certainly on the BBJ 1you have very little cargo space left.
For the fuel amount i believe you can check on the Boeing website.
Good luck