I’m not quite clear on your problem.
If you have: dock C to AB, where AB is a highly symmetric homodimer: just use regular docking, the symmetry is irrelevant.
If you have: dock C and D separately to AB, where C and D are the same and must dock symmetrically: this would be hard, but you could probably just dock only C and place D later, assuming C and D do not interact.
If you have: re-model the docking between A and B symmetrically when C is added to the system, that is probably hard.
Does that help any?