Well then you might need a 4 way, which I believe exist! Ive never done one, because the number of combinations and wires gets really complicated as you add different combinations! I am not sure what you are looking to do exactly. But if you are going to wall off a dedicated area for your office I would just put in my own light switch.
It sounds like it already has a light in it? I would just take that light off the existing switch and run a new switch to that light. That way the old 3 way does it job and the new 2 way you have added runs only your office?
Maps/diagrams/pictures help.
Sounds like you are competent enough with electrical to make this work. Again its your basement and you have access to the wires it should be pretty easy.