Adding a steering wheel:

A steering wheel can be exported from blender in the same way as other meshes. But you have to make sure that the steering wheel is centered at origin and facing towards negative x-axis as shown below.

Now follow the same steps as exporting other meshes like adding materials and assigning them to faces of the mesh for mFaces, applying all transforms and running the python script for exporting the mesh.

Now you have the '.msh' file, you can load it into body design panel using 'Load mesh' button . When loaded, it will be located at origin i.e. in the middle of rear axle at ground level facing negative x-axis as shown below.

As soon as you assign a node to steering point and steering column, the steering wheel mesh will automatically move to its position and orientation as shown below.