Here are the steps to merge the documents.
Open a new Microsoft Word 2007 document and type
Save the document to C:\Source.xml. Please change the “Save As Type” to “Word XML Document”.
Step 2: Open a new Microsoft Word 2007 document and type
Save the document to C:\ Destination.xml. Please change the “Save As Type” to “Word XML Document”.
Step 3: Open Source.xml in Visual Studio and add a package part to hold the content to be merged. Please note that content type is being used i.e. application/xml.
<pkg:part pkg:name="/word/embed/destination.xml" pkg:contentType="application/xml">
Step 4: In Source.xml, add a relationship in document.xml.rels section.
<Relationship Id="afChunkId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/aFChunk" Target="/word/embed/destination.xml"/>
Please note that value of the Target attribute should be the same as specified in step 3 for pkg:name attribute.
Step 5: Please specify the following element in document.xml. This element should appear where you would like to see the merged data.
<w:altChunk r:id="afChunkId1" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"/>
Step 6: Now open destination.xml, copy everything except the processing instructions and paste in between <pkg:xmlData> element specified in step 3 in Source.xml. Please note that you can also merge the Word 2003 XML document by just copying the XML except the processing instructions and pasting in between <pkg:xmlData> element.
Step 7: Save Source.xml and open it in Word and you will see that data is merged.
The following link explains as how Altchunk element can be used to merge the Open XML documents using Open XML SDK.