I'm not sure what good a plug-in would do, given the battery capacity. Even a fully charged battery (wouldn't charge over 80% anyway) and some sort of bypass to retard engagement of ICE (right down to 30% remaining battery juice), the car would be hobbled by distance and speed, unless you drastically increase the battery capacity and type. Additionally, power rating and kWh don't make for a very usable vehicle (though I forget the exact ratings). You do add the plug, upgrade the battery, and maybe upgrade the motor, but at the end, you would be trying to do an automaker's prototype program without an automaker's prototype budget.

I don't think HSD system will make a good plug-in system precisely because it was not designed as a plug-in system. You really need a plug-in system designed from ground-up, like Chevy's Volts. Oh, and some battery breakthrough.