Every object in the Thief world has a model. It is the shape (duh) and the texture/mesh color that is usually changed.
To change the shape of an object, GO: Properties>Add>Shape>Model Name. Now, for a simple test. Create a Key. I'm using Key7Orange. Now, open the item that is the model you want. Pretend you want to make the key look like a bowl, to make it look unimportant. Make a bowl. GO: Properties>WoodBowl>Shape. See that name next to Model Name? Write that down. Now delete the bowl. For the key GO: Properties>Add>Shape>Model Name. Then put in what the bowl's model name was, (mine was 'bowlwd' without the quotes), and exit out of the Properties. Now you've got a bowl that works as a key! Now, the sounds and the name of the key will be the same. Simply change those and you've got it made. You can make ANYTHING into ANYTHING! Make a wooden bowl into a tiara, or a sword into rubble! Have fun messing with Model Names!