Can I edit a closed ticket to add information?
No, tickets in a closed status cannot be edited. You can only edit tickets in solved status before they are closed through business rules, such as the Close ticket 4 days after status is set to solved.
Only follow-ups can be created from closed tickets. Ticket properties on closed tickets cannot be changed, furthermore, business rules such as triggers and automations don't act on closed tickets at all. For more information, see the article: About the inborn system ticket rules.
If your account uses the Zendesk Agent Workspace, you can redact closed tickets. For more information, see this article: Redacting ticket content in the Zendesk Agent Workspace.