I'm trying to use the Add feature to add a key file to a drive that is encrypted with a pass phrase. Maybe that's not possible.
It is possible but that is a bug in the plugin. I was able to create a container with a key file then add a key slot using a passphrase. The current code doesn't like how we pass the passphrase to LUKS to add a key file. I guess the best thing to do for now is create containers with a key file and always use a key file to add/remove keys. You can add a passphrase if you need one.