Yes, in space, but actually they'd need a lot of new ground units and buildings aside the satellites they'd need.
Furthermore, the 3 playable races will be perfectly balanced (hopefully^^), and a fourth would only ruin it.
And to repeat the reasons from the post above, it makes no sense to make them playable with only 2 ground units and 1 building, they need the same variety as the other 3 races, and this is, again, too much effort!