To import .nk2 files into Outlook 2010, follow these steps:
1. Make sure that the .nk2 file is in the following folder:
Note The .nk2 file must have the same name as your current Outlook 2010 profile. By default, the profile name is "Outlook."
To check the profile name, follow these steps:
a. Click Start, and then click Control Panel.
b. Double-click Mail.
c. In the Mail Setup dialog box, click Show Profiles.
2. Run outlook.exe /importnk2, This should import the .nk2 file into the Outlook 2010 profile.
After you import the .nk2 file, the contents of the file are merged into the existing nickname cache that is currently stored in your mailbox.
After the import is finished the .nk2 gets renamed with an . file name extension on the first start of Outlook 2010. Therefore, if you try to re-import the .nk2 file, remove the . file name extension.
more information please read: support.microsoft.com/kb/980542