Shared album question

Hello,

I am looking for a way to add a keyword or description to all the photos in a shared album. I can do this for the regular albums by selecting all the photos in the album, pressing cmd+I and then adding the text to the appropriate fields. What I ultimately want to do is select all the photos in my Photos app that are not included in a shared album and then delete them. However, I want to keep all the photos in the shared albums since they are shared with others. I have already exported all the photos I wish to keep out of Photos. Does anyone have any suggestions on how to do this?

Thanks!