Dudbaker,
See the RadarBox Preferences setting - File > Preferences > RadarBox > Transition Altitude.
Below the figure set in the top box, altitudes will be displayed if the correct QNH (pressure at sea level) is entered. So above the figure, Flight Levels related to the Standard Altimeter Setting 1013mb will be displayed. Mode C always shows the level relative to 1013 mb. RadarBox does the maths to show altitude.
If the airfield elevation is 500ft amsl, with the correct QNH set, the aircraft will show 500 ft as it touches down.
If you are connected to the Internet, you can set the QNH automatically using the Select Airport in the SmartView tab.