LDraw is a popular CAD format for sharing 3D LEGO models. The simulator is capable of parsing an imported (*.ldr) file, and then assigning physical attributes to it such as weight, centre of mass, and collision data to each model element.
If you haven’t already done so, you should consider downloading and installing the LDraw "All in One" installer, as it will greatly expand the number of bricks that the simulator can work with.
After installing the ’All-in-One’ library, you will also want to update LEGO Digital Designer so that you can export your models to the sim. This can be achieved by clicking on Help menu and selecting Patch LEGO Digital Designer.
Applying the patch for LEGO Digital Designer