How to Move and Merge Microsoft Outlook Autocomplete (.nk2 and .dat) Files
Screenshot missing - will replace soon.
This is the auto-complete feature in Microsoft Outlook. As you can see, I have only typed
ti and it is displaying an email which I may want.
These emails are stored in a file which Outlook has created however when one reinstalls Windows or upgrades to a more recent version of MS Office, this file may be lost.
This .nk2 or .dat file can be located in:
XP: drive:\Documents and Settings\<user>\Application Data\Microsoft\Outlook
Vista: drive:\Users\<user>\AppData\Roaming\Microsoft\Outlook
Windows 7 drive:\Users\<user>\AppData\Roaming\Microsoft\Outlook

Note
This tutorial will show you how you can merge or transfer your Outlook autocomplete email lists. Should you reinstall Windows or upgrade to a newer version of Office, you will need to do this if you want to keep the list of emails that are shown in the autocomplete form
What happened to me was I upgraded to Office 2010 and I was using Outlook 2010 for a while before I remembered that I hadn't transferred the .nk2 file. So I was left with a .nk2 file and a .dat file, both of which I needed the contents of.

Warning
- Make sure that you have closed Microsoft Outlook before you continue.
- Please make sure you make a backup of the .nk2/.dat file before you alter it in any way

Warning
Make sure that you only open these files in NK2Edit by using the File > Open .NK2 File. Do NOT open them by double clicking on the file in Windows Explorer.
1. First of all, we are going to need to get the .nk2 files that we need. The location of your .nk2 file will vary depending on your OS (Operating System) and version of Office. The table below should explain where you can expect to find your .nk2 file:

NOTE: I would appreciate it if you guys can confirm these locations, I think they are right but I am not sure.

Note
In Office 2010, the file is called something along the lines of Stream_Autocomplete_0_188E43DE2C399A47B227874D14929BC7.dat (yours will not be the same, this is what mine was).

2. Now you've found the .nk2/.dat file we need to open it up in NK2Editor:Here is the website of it: NK2Edit: Edit AutoComplete files (.NK2) of Microsoft Outlook
This little program is portable and doesn't need installing. So download it and extract the files from the .zip file. There should be one called NK2Edit.exe.

Note
I will not be able to provide screenshots from now on because my .nk2 file has my personal contacts in it and I don't think that they would appreciate me putting their emails on the internet.
3. You will need to open one instance of NK2Edit per file you wish to merge - the merging of 3 files requires 3 windows. Resize the windows so you can clearly see each window on your screen. Now open the files in their corresponding windows by pressing File > Open .NK2 File. 4. In one of the windows, select all of the entries in the .nk2 file by pressing Ctrl+A. Then you will need to right click on them and select Copy. 5. Now we need to switch over to the other window and press Ctrl+V to paste them. You should now see the contents of both .nk2/.dat files in the same list 6. Now you will need to save the new .nk2/.dat file. 
Warning
Make sure you have backed up the .nk2/.dat file that is currently in use before you proceed
Press File>Save As... and then chose the type of file that is currently in use:
For Office 2003/2007 use: Outlook .NK2 files (*.nk2)
For Office 2010 use: Outlook 2010 AutoComplete .dat file (Stream_AutoComplete*.dat)
You will need to save this in the location that you found the .nk2/.dat file in the first place (see table at start). Make sure that you save it in the correct place:the same place that the current .nk2/.dat file is located in.
Your old .nk2/.dat file will still be there, edit the name of the file and add BACKUP to the end of the name. You will need to save your newly created .nk2/.dat file AS THE SAME NAME AS THE OLD .NK2/.DAT FILE WAS CALLED (before you added backup to the end). For example
Say you took a .nk2 file from one computer, lets call it aaa.nk2, and wanted to merge it with a .dat file on your computer, lets call it bbb.dat. You would need to open them up, merge the two files. Change the name of bbb.dat to bbb BACKUP.dat and you will need to save the merged list of contacts as a .dat file, with the same name of whatever bbb.dat was called and in the same place as bbb.dat was found.
Then reboot your computer and open outlook, your autocomplete list should now contain entries from both files.
Hope this helps guys!
Tom
If you need any further information, or you don't like the way I've done this tutorial, give me a shout and I'll see what I can do or you can go to:
How to merge 2 or more NK2 files
And follow their instructions