The best way I could explain it would be; study the hatch engine file, observe how the turbocharger section is written and defined. An easy way would be to copy paste the hatch 'Turbo' to the D-series engine file, then edit it to match the pickups criteria. And then add the slot so it's selectable. If you want, I can do the turbo addition for you and upload the file. - - - Updated - - - It was easier than I thought, actually. So here you go. I had just enough time on the desktop to edit the file, launch the game and see if it was selectable (gf needed the computer for work). It is, and it works. I had to edit the power values blind, so I don't know if it's too powerful or not enough. It was a small change, so it should have more 'pep', but nothing extreme. To clarify: This is the pickup V8 engine file, with a selectable "Stage 1" turbo added to it. That is all. (There's a possibility that the transmission won't like the extra power, and won't put all of it to the tires. I can't test it right now so you may have to mess with it, if you know how)
I think beam NG should have a turbo charging option for all vehicles with select engines. and maybe a supercharging option as well.
We now have real turbos that can be defined in the engine.jbeam. (read the date on the OP this thread was from 2013)