That center is a full-range speaker; to the extent it can play anything, it's still considered full-range, likely around 400hz to 10 kHz.
As for adding mids down the road, not that simple.
2-way versus 3-way means different capability requirements for the components, different crossover points, etc. Just adding a midrange to an existing 2-way component set without changing the frequencies (crossovers) that they are receiving signal from means you are adding overlapping (doubling) output at some frequencies, creating nulls or phase cancellations at other frequencies, very undesirable.
3-way done correctly does allow one to raise the front stage much higher, front and center of the listening area.
That said, it has a lot to do with not only the crossover points, but it's also due to their physical location, the beaming characteristics of a given-size driver you intend to use, its location, lobing, phase corrections at the crossover points, etc.