This tool is designed to decrypt an encrypted SSH private key and load it into a running ssh-agent (similar to ssh-add functionality), with the added twist of getting the passphrase from the OSX Keychain.
This functionality (ssh-add -K) existed prior to OSX Sierra; now it depends on custom .ssh/config settings instead; perhaps you run a non-Apple version of SSH, perhaps you like to share your .ssh/config as part of a dotfiles repo - either way, the custom config options won't work for you. This tool is designed to help that.
Currently super experimental (is your environment exactly like mine?), but functional. Needs a lot more work. Loading the key into the Keychain is an exercise left up to the careful reader. The command line tool security is your friend.