It's possible to convert JPEG's to DNG in Lightroom. Is it possible to extract the original JPEG again after conversion (a 1:1 identical copy to the originally imported one) later on?
Not as such, no. The DNG will contain essentially a pixel-for-pixel version of the rendered version of the JPEG you imported. That is, the TIFF image inside the DNG will be identical to the original JPEG if you were to view the two side by side (or if you were to do, say, an image subtraction, you would be left with no difference). However, to create a JPEG from the stored image, it needs to be compressed again, and JPEG compression is lossy. If you choose a very high quality JPEG, you would need to compare the images bit to bit to notice the difference (you won't perceive anything), but if you need to compress much at all to get the "same" file, there will be differences.