A TextFieldMaster can only be assigned once.

 // Create a fieldmaster for our newly created User Text field, and access it's
 // XPropertySet interface
 XPropertySet xMasterPropSet = (XPropertySet) UnoRuntime.queryInterface( 
 XPropertySet.class, mxDocFactory.createInstance(
 "com.sun.star.text.FieldMaster.User"));
 // Set the name and value of the FieldMaster
 xMasterPropSet.setPropertyValue ("Name", "UserEmperor");
 xMasterPropSet.setPropertyValue ("Value", new Integer(42));
 // Attach the field master to the user field
 xUserField.attachTextFieldMaster (xMasterPropSet);
 // Move the cursor to the end of the document
 mxDocCursor.gotoEnd(false);
 // insert a paragraph break using the XSimpleText interface
 mxDocText.insertControlCharacter(
 mxDocCursor, ControlCharacter.PARAGRAPH_BREAK, false);
 // Insert the user field at the end of the document
 mxDocText.insertTextContent(mxDocText.getEnd(), xUserField, false);