Something I tried on a "need a lot of water" hike was to buy a couple of platypus units that clipped onto my waist belt, each holding a liter of water --- for just that reason (pack not rated to take lots of weight). Something like this: https://www.rei.com/product/768122/platypus-holster-bottle-carrier-1-liter
I can't recommmend the specific units I used, but they were different than the one I've linked to above. Maybe platypus learned something in the interim (?). My version had a sort of clip to hold it onto the waist belt, and in general the problem came when I tried to set the pack down or sat down on something with the pack on. The units hung down below the waist belt, of course, but with the little clips what would happen is that they would just come off the waist belt, and it was a bit of a PITA each time to try to get them back on. Ultimately I just carried them strapped to the pack anyway, and then just got rid of them.
*Maybe* these units would work better, as there's a continuous loop rather than a clip. But even still, whenever you take the pack off, they'll want to shift and slide off the hip belt ends, and you still really can't sit with these, unless you happen to find a rock or log that allows the units to hang down on each side when you do.
So maybe not such a good idea, but --- maybe it would work for you anyway, just so long as you're aware of the downside.
FWIW, adding more weight than rated generally worked fine for me with various backpacks, at least for limited distance/time, and water weight is exactly that --- you load up at a water source, then drink it down so the weight declines relatively rapidly.
Best of luck!