The 225 is derived from the An-124, which has 10 integral fuel tanks in the wings. Comparing the specs, it looks like fuel capacity is the same as the An-124, so the center section of the wing (the only new part of the wing) doesn't contain fuel.
Based on that, I think the tanks are in sections 2-6 and 13-17. I'm assuming the outer wing sections 1 and 18 are ...
With straight wings the root extension method is very common. Airplanes that come to mind are
Me-109 H (high altitude version with increased span)
Beech Twin Bonanza and Model 65 Queen Air (used the Bonanza wing from the engines out)
SB-10 (uses SB-9 wings stuck to a new carbon-epoxy center wing).
This is impractical with swept wings because of the strong ...