Yes you can do this in the print module.
In the settings under Print Job select Print to: JPEG File. Select Custom Package and you should have a blank canvas. Then drag your image onto the blank page and resize as needed. Its not as robust as doing it in photo-shop, but works well enough.
and as another poster mentioned you could get this accomplished in Effects -> Post Crop Vignetting in Develop module as well, but you wont get a sharp border and can only get it black or white. In the Print module you could select any color you want for the background.
Hope this helps!