Hi Rob
Unfortunately, once a case is closed/resolved, you cant modify it. So, we cannot add/update fields after closure. But there are few alternatives to help you:
1. If you can add, 'modified on' to your 'Resolved cases ' views, it will act as case closed on date & time (For resolved cases, you to consider modified on as closed date)
2. You can create a personal or system view, to see the actual end of your case:
Go to advanced find>>look for activities>>second row: set activity type=case resolution>>click on 'edit column' on the top>>select few columns from case resolution activity like- subject, actual end, billable time, total time resolution and then change record type to 'regarding case'>>add whatever columns are required like-customer, created on, owner etc.>>hit save on the top
following screenshots will guide you:
This is how you can create a personal view, which would be visible only to you, if you need help on creating a system view (visible to all users), do let me know
3. You will have to create a plugin, see this thread: [View:https://community.dynamics.com/crm/f/117/t/120835:750:50]