Lightroom 3 doesn’t have a dedicated built-in feature for permanently adding pillarbox or letterbox bars to an image in the normal editing workflow. Two workable Lightroom-only options were suggested:

1. Print module workaround: Create a custom page size, set the page/background color to black, and place the image on it. This is useful if you want the final exported output to include black borders.

2. Graduated Filter workaround in Develop: Add a Graduated Filter from the edge, reduce Exposure/Brightness fully, and compress the gradient until it becomes a near-hard edge. Repeat on other sides as needed to simulate black bars.

So, no native one-click “add bars” tool was identified, and no specific free plugin was confirmed in the answers. If you need precise, permanent borders as part of the image file, Photoshop or another editor is still the cleaner option; if you want to stay in Lightroom, the Print module is likely the most practical method.