Depending on the age and type of box you may be able to attach another device box to it. The old box needs to be an expandable type with screws holding the sides on. The dimensions for the screw placements have moved around over the years, so it could be hard to find a compatible box to attach. You will need to make a larger opening and do some wall repair to get the new gang in and attached. I would remove the old box and move the cables into a replacement 3-gang old work box. If done very carefully the hole in the wall for the new box will be the correct size and no wall repair will be required.

Eric, please don't take this the wrong way, I'm not trying to be a wiseacre. Randomly connecting stuff to various pairs of wires isn't going to solve your problem. The basic problem here is that you don't have quite enough basic electrical background yet. Two errands. First, go to the hardware store and/or cheap tools store and get yourself some basic test gear. You need a DMM (digital multimeter) that can measure resistance (continuity). Also get a rubber light bulb socket with wire pigtails, and a 7 watt night light bulb that fits it. (The links are for illustration only, and are not meant to be endorsements.) You'll see how to use these after the next step..... Which is, go to your library and check out one or two books on basic electrical wiring for the homeowner. (Ask the librarian for help if necessary.) Read them all the way through. When you read about how switch loops work, I bet the light bulb will go on. (Sorry, bad pun.) With your new tools and quick education, you should be able to figure out which wire goes to what. If not, hop on back, and we'll see if we can help. Electrical wiring isn't all that complicated, it just has certain conventions that you need to learn. Good luck! Let us know how it goes.

Your description is a bit confusing, but what you need to do connect the white from the light to the white from the incoming power cable, the black from the light to one screw of the switch, and the black from the incoming power to the other screw of the switch.

bus driver: yes, indeed, it's plaster over wood lath. we just paid to have the wall with metal box in question smooth coated... so, as minimal as possible so I don't have to kick myself in the butt. However, adding the three way switch and new light fixture will improve lighting and functionality of the house, so I will consider :) hendricus: yes, plastic boxes and Romex are allowed. I'm in Southern California.