drag-and-drop prefab assets into the scene during gameplay?

Is it possible to create a virtual-world sort of game in Unity3d where the player can drag-and-drop prefab assets into the scene during gameplay? For example, in most Zynga games like Farmville and Cityville , the player can select an object (such as a building) and drag it onto an empty space on his/her farm or city. The player can also move that object around during gameplay. Is it possible to do something like this using Unity3d or must all assets be placed during the creation stage?