Archiving Mac Mail for later retrieval

I have several users who insist on saving emails stored in Mac Mail to their local hard drive. If they drag messages to the local folder, the emails are saved with a .eml extensions. They can click on the file later and it open and appear as the original email did in Mac Mail. The other option is some of them use File/Save As to save them to the local drive. If they don't select "Raw Message Source" when saving, the files saved as Plain or Rich Text. When that happens, the emails open unformatted in TextEdit. I have instructed them to save these emails properly moving foward. However, one of our big wigs has quite a few emails that are no longer in Mac Mail that he saved to his local drive as Plain or Rich Text. Is there a way to convert those to .eml files so that they open back up in Mac Mail properly. I know it is not as simple as just changing the file extension.

Thanks for any help.