There’s a lengthy thread over at Adobe’s Lightroom feature request and bugs forum Is there a way to re-associate converted DNG files with the original RAW files? The user had converted files to DNG, but then decided he wanted to switch back to proprietary raw files (he’d got them all in his backup) and didn’t know how to make the DNGs’ thumbnails in Lightroom point to the raw files.
Most of the thread is about one solution – learning SQL and updating the database. Fun if you’ve got the time / inclination.
The other solution is somewhat easier – my Syncomatic plug-in. All you would do is get the raw files into your catalogue, and tell Syncomatic to copy metadata and adjustments from DNGs to raws.