Adding an iCloud email to existing storage plan

I have upgraded my iCloud storage and have plenty of space. I also have two other iCloud emails which I set up years ago and have been using with no problems. Now it’s is saying that one of my emails is full and doesn’t have enough storage. I can’t see how to add it to my already HUGE storage plan. I’ve tried adding as a family member but that hasn’t worked. What can I do?

[Re-Titled by Moderator]