It's possible, within certain limitations.
You're going to have to build the car out of pre-molded parts in the game.
You can't model, animate, load and distribute models to clients on the fly, at least not without seriously editing the engine.
You're going to get the old "hey all of my body parts are lagging 3 feet behind me!" effect you got in RTB...
Unless you design the vehicle the same way Badspot made the avatars (which is coincidentally why we can't make custom avatar parts) and make a default vehicle consisting of every part overlapping and just hide the parts you aren't using.
But when it comes down to it, there are things that Roblox will probably always do better than Blockland. The Roblox engine was designed from the beginnings as a very physics oriented engine. If you've taken a look at other Torque games, you'll quickly notice that Torque isn't very robust when it comes to physics. In the few games that do have "objects" with physics, the "objects" are typically just heavily edited vehicles.
Your suggestions have been intelligent though, don't let anyone in this thread discourage you. But also keep in mind, we don't need to compete with Roblox. People who buy another game aren't necessarily "lost customers" in the same way that someone who buys one brand popcorn over another is. All playing Age of Empires II did was lead me to purchase Alpha Centauri, Civilization, World in Conflict, Age of Mythology, Universe at War, Rise of Nations, Red Alert 2... (not in that order or at the same time of course)