If you want to use a shared account when doing a mail merge, create a new mail profile for that shared account.  You must be an owner of that shared account to reference it when creating a new mail profile.  You will open the new mail profile created below when you want to do the mail merge from the shared account.  Then you will switch back to your email profile to interact with your personal mailbox.

Instructions for doing a mail merge in Word are available here.

1. Open Outlook.
   
   
2. Choose the File tab in the ribbon.
   
   
3. Choose Account Settings, then select Manage Profiles from the menu.
   
   
4. Click Show Profiles
   
   
5. Click Add
   
   
6. Provide a descriptive name, and click OK.
   
   
7. Select Manual setup or additional server types.
   
   
8. Enter the email address of the shared account where you are an owner, and click Next.
   
   
9. Click Finish.
   
   
10. Select Prompt for a profile to be used, and click OK.
    
    
11. Click OK
    
    
12. Close and reopen Outlook.
    
    
13. Select the new mail profile you created so that you can complete the mail merge using that shared account.
    
    
14. After completing the mail merge, close and reopen Outlook to select your mail profile.