A mailmerge main document can use an INCLUDETEXT field to incorporate another document, whether absolutely or conditionally. No VBA is required for this - just field coding.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Well the problem here actually is that the main document is not a mail merge document. Instead it should be trying to append a document that in fact is a mail merge document. And upon appending it the mail merge doc should be up to the latest changes from the data source.