Plus > Preferences > Conversations > Log Encryption > "Re-Encyption"
I'm not TOTALLY sure its this function as I don't use it regularly. But it's your best shot.
As an alternative which has the function to Encrypt seperate files or multiple ones use the script Log Manager found here.
http://shoutbox.menthix.net/showthread.php?tid=77116
quote:
Originally posted by Mattike
...encryption actions - By the use of a user-friendly window, you can choose files to encrypt, decrypt or change the password of. It shows you basic information about the files, such as file size and last modification time, and lets you set the options for the process, such as your password and the output folder. To prevent you from accidentally hitting the Start button, a confirmation first must be accepted before the process starts. Then, you can see the progress of the action by the use of progress bars and in the status column of the files list.