As a certified ETC technician, if you truly are talking about ETC Link, I am going to have to say not to do what was described above. In addition to DATA There is also power running down the ETC link line either coming from the console, or coming from the dimmer rack - and if you plug DMX wiring into it - even if it does fit - then you run the risk of damaging your robo colors. To top it off, there is an ETC Link terminator that is inline somewhere that could be damaged as well.
ETC Link is provided mainly for dimmer monitoring, but you can also have an ETC link Station that will recall presets that are recorded into the CEM. The dimmer monitoring can be very simple (included with every system that has link - i.e. rack voltages and dimmer room temperature) to very complex (requiring the use of AF dimmer racks and AF dimmers - where you get actual load recording and it can tell you if your lamp had blown).
You need to be more specific about what your black boxes are before anyone can tell you what to do. You probably are just talking about a surface mounted ETC provided back box for the DMX1/DMX2/ETC Link faceplate, if I had to guess, and not an actual node of any sorts.
You can have 2 port nodes running on an ETC NET1 network (the network type that an Express puts out naitively), but you would also have an LCD screen associated with that, and I havent heard you mention an LCD screen.
Again the long and the short of it is, dont use the ETC link wiring for anything other than ETC Link. Is it possible for it to work? Yes, but if you damage your fixtures or anything else becuase of it, it would not be covered under any sort of warranty and could cost you up to a couple of thousand dollars in repair work.