You may want to combine the Outlook Inbox of different accounts, into a single file, if you want to arrange the email accounts in your Outlook. When you combine Outlook inbox, you also save on-screen space. If you use the automatic method to create email accounts in Outlook 2007 and later versions, by default, each email account will prompt Outlook to create a new file and hence, a different inbox. You can always combine the accounts into a single file so that it becomes easy for you to manage your emails.
This is how you can achieve it!
Combine Outlook Inbox
Note: This procedure assumes POP3 accounts.
If you are creating new accounts using the auto-detect feature, you can combine the inbox, once you are done with creating them. If you have already created the accounts, you can still combine them in Outlook 2007 and Outlook 2010.

To do so:
1. Open Microsoft Outlook and click on File menu.
2. In the
File menu, click on
Account Settings and in the drop down menu that appears, select Settings again.
3. You will be presented with the
Account Settings window that lists all of your existing email accounts. Make sure you are on the
Email tab
4. Click on the email account whose inbox you wish to combine. MS Outlook will present you with
Change Folder option towards the bottom of the
Account Settings window (See Fig below).
5. In the Change Folder dialog box, select
Outlook and then
Inbox. If you wish the mail to be delivered to a custom folder, click
New Folder to create a new folder. In case you wish to use a new PST file for email, you can select
New Outlook File. But since your contacts, calendar etc are already stored in outlook.pst, it is better to select
Outlook ->
Inbox as it will save time on backing up files (See last figure below).
6. Once you have selected the folder you wish, click
OK.
7. Repeat steps 4 to 7 for each email account inbox you want to combine.
8. Close the Account Settings window

Following the above procedure, you can close the additional files that were created by MS Outlook for different email accounts that you have now combined as the new mail will be delivered to the folder you selected in step 5.

There it is, you should now have your accounts merged in your Outlook inbox in versions 2007 and version 2010.
source:thewindowsclub