When the wiring is in, the black box will not have the slightest idea what to do with the information. It has no plughole labelled 'Additional Sensors'. I have a bad feeling it would not even have the capability of being modified to take a new input.
It is possible to daisy chain data. There does not need to be a one to one relationship between sensor and input. You simply offset the values from one sensor by a known constant so it can be decoded later (in software) as coming from a separate source. This would be a major weight saving too. Acceptable to the FAA? Who knows...