There have been several requests that the Enigmail add-on for Thunderbird support encrypting the existing contents of the inbox. It didn't make it to either the "developers to-do list" or the "feature wishlist for v2.0".
https://sourceforge.net/p/enigmail/f...read/b0940ad0/
There is a IMAPCrypt utility that supposedly lets you encrypt the contents of an existing IMAP folder using Gnu Privacy Guard (GPG). I ran across another project that uses gpgit to do the encryption and pipe2imap to upload the encrypted message.
https://www.chrisleephd.us/projects/imapcrypt.html
https://www.grepular.com/Automatical...g_Email_Part_2