|This is the last time your account was accessed.|
I believe all you can do is delete and add a new one.
I had the same question. During the process to remove the seconday e-mail address it warned that it could not be added later. I needed to remove it to solve a technical glitch per verizon but now I don't think I can add one going forward.
As verizon no longer offers email, they may have changed the way to add secondary accounts.
Log in to Verizon website.
I see sub accounts under My Verizon Profile.