How to cache pass phrases into OSX keychain for existing PGP disks

I am using Ecryption Desktop for Mav version 10.3.2 MP1 build 15337 on OS X Mavericks 10.9.3

I would like to store my PGP passphrases on the OS X Keychain but I have only found that option when creating a NEW virtual disk; during the process there is a box that reads "Save passphrase in Keychain"

However, I have many other EXISTING disks that do not show the "Save passphrase in Keychain" option while opening them. 

Any guidance is appreciated