I'm not sure which version of Outlook you're using. I'm using Office 365 and use the Windows Application Menu.
1. Focus on a message in the message list
2. Open the Windows Application Menu (WINDOWS APPLICATION KEY or SHIFT-F10)
3. Press ENTER on Follow-up
4. Within he Follow-up menu, press ENTER on "No Date"
After setting the Follow-up, set your email arrangement to "Flag Status."
This puts your flagged messages at the top and your other messages sorted by date at the bottom.
Messages with "No Date" follow-ups are always at the top.
To arrange messages by Flag Status:
1. Open the View Ribbon (ALT-V)
2. Activate the "Arrange By" (AB)
3. Within the list, select "Flag Status" (L)
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