Take the F3000 as an example. In the gfx.cfg under "chassis" and "cockpit" there is "steer" and "name = B05_SWHEEL".
In your 3d model the steering wheel needs to be one object with one of its local coordinate axes as the rotation axis. Put it's name behind "name" in the gfx.cfg.
For the wheels look for "lf_wheel", "rf_wheel" and so on in the gfx.cfg.
The wheel needs to be in the coordinate center. Put the name of your wheel .edf behind "model" / "file =".