You may want to try one of the Many-to-One Mail Merge add-ins, from:
Graham Mayor at
http://www.gmayor.com/ManyToOne.htm; or
Doug Robbins at
http://bit.ly/1hduSCB
In addition to a 'Many to One' merge, the latter handles:
• Merge with Charts
• Duplex Merge
• Merge with FormFields
• Merge with Attachments
• Merge to Individual Documents
• Merge, Print and Staple