More info, please. In what format are the e-mails on your thumb drive? Into which mail application are you trying to import?
An e-mail does not usually live as a separate file outside of some sort of mail application, unless it is exported as a PDF or text file. Once exported like that I don't think they will come back into a mail app as an e-mail; as an attachment in an e-mail, but not as an original e-mail.
An exception to the above would be if the e-mails were exported in some mailbox format. Is that what you have? If so, then knowing the source and destination applications would be helpful.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!