Adding an Attachment to Mail Merge in Outlook

Unfortunately it is not possible to add an attachment to a mail merge with Outlook.  

However, there are two options for a workaround:

Upload the attachment to Box, and create a link to the attachment. 

Proceed with the mail merge, but copy and paste Box shared link into the body of the email

          OR

Do a regular email message and BCC everyone with attachment. Make sure all email addresses are separated by a semicolon. There is a limit of a few hundred in the BCC line though. (Note merge fields will not be possible this way)