If I have a SOAPMessage constructed using the MessageFactory interface, I should be able to import a node from a different document and append to the first child of soap body.

This does not work correctly. I have tested this against the SAAJ RI and it works fine there.

Here is sample code:
MessageFactory factory = MessageFactory.newInstance();
File soapreqfile = new File("soap-request-template.xml");
SOAPMessage msg = factory.createMessage(null, new FileInputStream(soapreqfile));

//Get the node that needs to be imported
Node someNode = getNode("import-node.xml");

//Import the node
Node importedNode = msg.getSOAPPart().getOwnerDocument().importNode(someNode, true);
//Append the node to the first child
msg.getSOAPBody.getFirstChild().appendChild(importedNode);