The compressed folders support can also be used to handle encrypted folders. If you want to encrypt a folder with PGP, you may want to use the following hooks:
open-hook \\.pgp$ "pgp -f < %f > %t" close-hook \\.pgp$ "pgp -fe YourPgpUserIdOrKeyId < %t > %f"
Please note, that PGP does not support appending to an encrypted folder, so there is no append-hook defined.
Note: the folder is temporary stored decrypted in the /tmp directory, where it can be read by your system administrator. So think about the security aspects of this.