Looking at the picture...if the rucksack is not attached, you have a little easier job.
Either way, you're going to have to rebuild the back to remove the "weight" of the sack and add the back of the load bearing equipment suspenders. Next you'll need to carefully carve away the shoulder straps and rebuild those area to match the rest of the clothing and not show any of the weight efftect on the clothing folds.
Expect a lot of carving, sanding, puttying, sanding some more and repetition of those steps before you can expect to have a good torso.