Get large horse syringe for big gauge needles, discard needle, bore hole top and bottom, inject epoxy into the bottom until it comes out the top, plug the bottom with tape. Epoxy would fill the voids but will also soak into the balsa so will stand "topping up" as the "pour" goes on. At the end you hope to have filled the void enitirely.

I've never done this on a hull though have on 'glass cabin sides with success. Have also done it for delaminations in ply decks. It's remarkable how much hydraulic pressure you can generate with that syringe. You actually have to be carefull to only fill a void and not create other problems like lumps or raises of material.