If the question is "how do I retroactively embed XMP into DNG", there are two options:
1. Turn on the Catalog Settings option to "Automatically write changes into XMP". That will cause XMP metadata to be written to ALL images in the library that currently have the Metadata Status showing as "Has been changed", then once the backlog has been done it will continue to update the XMP for all images that are subsequently changed.
2. You can do a one-time update of the XMP for selected files only....select the images, then do Cmd+S. That updates the XMP for the selected files, but does not automatically continue to update them (you need to do option 1 for that).