Something like this:
foreach (XmlNode node in documentB.DocumentElement.ChildNodes)
{
XmlNode imported = documentA.ImportNode(node, true);
documentA.DocumentElement.AppendChild(imported);
}
Note that this ignores the document element itself of document B - so if that has a different element name, or attributes you want to copy over, you'll need to work out exactly what you want to do.
EDIT: If, as per your comment, you want to embed the whole of document B within document A, that's relatively easy:
XmlNode importedDocument = documentA.ImportNode(documentB.DocumentElement, true);
documentA.DocumentElement.AppendChild(importedDocument);
This will still ignore things like the XML declaration of document B if there is one - I don't know what would happen if you tried to import the document itself as a node of a different document, and it included an XML declaration... but I suspect this will do what you want.
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