To duplicate it exactly, you'd need to understand exactly what the camera does between the sensor and the finished JPEG. I'm sure Nikon knows, but I'm also sure they're not telling beyond what they cover in the owner's manual. You can, with some experimentation, approximate it. There are two major categories of changes you need to deal with:
The first is how the camera enhances (or attempts to enhance) the image after it has been captured. This normally involves setting white balance and applying some combination of adjustments to sharpening, contrast, brightness, hue and saturation. Your best bet would be to work with a few raw images and their camera-produced JPEGs side-by-side to see if you can find a set of basic changes that work.
Nikon's more recent bodies have a feature called D-Lighting that lightens the darker areas in the image. This is the same thing done by the Fill Light control on some processing software does. There's an "Active" flavor of this feature that adjusts the camera's exposure program for maximum dynamic range after the processing happens. You won't be able to compensate for this after the fact because you can't change the exposure.
The second is compression of the finished image into a JPEG, which happens after all of the processing outlined above happens. You can get an idea of how much compression is applied by setting your camera to save a RAW and JPEG for each image, shooting some pictures and converting the RAW to a JPEG using your processing software. Keep changing the compression level until the processed JPEG is about the same size as the one produced by the camera.
Jrista's comment covers the batch processing issue pretty well. Most of the major programs have an "apply settings and export in batch" feature.
If you haven't done much with raw images, I'd encourage you to spend some time tinkering with them using a good processing program (LightRoom, Bibble/AfterShot, etc.). You can get quite a bit more out of raw files with minimal work, and the results can be much more rewarding.