Following mGalaxy's tutorial, I created a new system entry for Turbografx-16.
This is great if you would like to keep PC-Engine (JAP) and Turbografx-16 (USA) separated, like the Hyperlist databases.
The icon.png could use some work, it's good enough but not exactly like the default systems.
This is the XML code, paste this in the BaseSystems.xml file in the Data/BaseSystems folder.
<System type="Console">
<Name em="NEC_TurboGrafx-16" tgdb="TurboGrafx 16" gb="55|TG16">NEC TurboGrafx-16</Name>
<Emu selected="0">
<Cmd id="0" hidden="false" name="MagicEngine" value=""%path\%file%ext""/>
<Cmd id="1" hidden="false" name="Mednafen" value="-fs 1 -sound.volume %volume(0,100) "%path\%file%ext""/>
<Cmd id="2" hidden="true" name="RetroArch [mednafen_pce_fast_libretro.dll]" value="-f -L cores\mednafen_pce_fast_libretro.dll "%path\%file%ext""/>
</Emu>
<Extensions>.pce,.zip</Extensions>
</System>