Proposing the idea of Fortress Walls. The Supply Depot is a good unit to envision.
Is it at all possible to create a unit that can be built upon like terrain? This would allow players to create walls that can extend 2 or 3 units high.
In any case, I think it would take a mix of data and triggers. If not, I assume that I could just create my own 'stacked' unit that is a single actual unit, and when a play builds a supply depot on top of a supply depot, he'll get that new unit in its place instead.