You can add as many ground wires as you want to at any place you want to. There's nothing that says you have to connect it only where the original ones are.

You won't find a picture or drawing in the service manual. We just look for the ground wires until we find them. The main one is the larger negative battery cable connected to the engine block. The smaller negative battery wire bolts to the body. Any other ground wires are for noise suppression for the radio and to prevent interference with computer sensor signals.