I don't think John's Syncomatic route would be one-by-one, but I could be wrong. Although it wouldn't be free.. Unless it wouldn't be too slow to do 10 at a time with the unregistered version.
Assuming the names would be the same with the exception of the file extension:
One roundabout [PITA] way would be to use the dng converter on copies to convert said images to DNG, then make sure those DNG's are reconnected in LR. Then you could use the late Rob Cole's xEmP plugin. Then select those DNG's and use his plugin to write out the XMP's. Remove (not delete) those files in LR and then copy over the original proprietary raw versions.. And then reimport with them being along-side the XMP's you just wrote out with the plugin.
Obviously make sure everything is backed up properly before trying all this. But I'm thinking it should work.
I tried attaching the plugin, but it's about 5.1MB as a 7zip file, which turned out to be too large to attach. However, if the above would be something you might be interested in, let me know and I should be able to put it on a public Google Drive for download.
OR... Just live with those particular files being DNG and be done with the matter.
Sticking to proprietary going forward.