I don’t believe that’s true. I’ve seen reported that any ticket (besides an AP) can have Genie+ added for length of stay either through the app / online or by calling ahead. I’ve never heard anything about a package restriction.
However, you can’t add Genie+ to just a few days of your ticket in advance. It’s all or nothing. If you don’t do length of stay in advance, you have to do one day at a time day of for whichever days you want it as they come.