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Outlook 2010 doesn't store autocomplete suggestions in .NK2 files like older versions of Outlook. It stores the info in a hidden record in the PST file.
Session → Logon → Mailbox → Inbox
Right-click Inbox → IPM.Configuration.Autocomplete
Subject → Right-click IPM.Configuration.Autocomplete
Export Message → MSGFile Unicode
Record must be deleted before importing.
Session → Logon
Folder → Import → From MSG