They didn't need to, they made the game in their program software and can probably script and object into the Lot or game where they want it.
Actually, judging by the fact that the shells to high rises are available in the buyDebug menu, I'm relatively certain that they didn't program every individual lot from scratch. I am also aware that there are codes that they have access to in their design version of the program that are removed from the retail version of the game. And I understand why they remove some of them...I would imagine that multiple mailboxes on a lot would wreak havoc on a game.
Not really because how are you going to get that Mailbox off the 10x10 Lot? You would need a bigger lot and I think I have just the Solution for you. (I just have to go into my game to double check for you!)
You don't need to get it off the lot. You can place any lot in your bin into any existing lot in the game that is the same size or larger and is of the same type. The lot remains the size of the lot in the game, and the smaller lot is centered in the larger lot when it's placed. However, you can't place a larger lot from your bin into a smaller game lot for obvious reasons.
Thus, you could drop this lot into any residential lot and it would fit. Of course, there's still the problem of the multiple mailbox (possible) issue that I mentioned above, so I guess that this wouldn't be as easy a fix as I thought. Darn it, I thought I was onto something lol.