XNode.AddAfterSelf Method (Object[])

public void AddAfterSelf(
	params Object[] content
)

XElement srcTree = new XElement("Root",
    new XElement("Element1", 1),
    new XElement("Element2", 2),
    new XElement("Element3", 3),
    new XElement("Element4", 4),
    new XElement("Element5", 5)
);
XElement xmlTree = new XElement("Root",
    new XElement("Child1", 1),
    new XElement("Child2", 2),
    new XElement("Child3", 3),
    new XElement("Child4", 4),
    new XElement("Child5", 5)
);
XElement child1 = xmlTree.Element("Child1");
child1.AddAfterSelf(
    from el in srcTree.Elements()
    where (int)el > 3
    select el
);
Console.WriteLine(xmlTree);

<Root>
  <Child1>1</Child1>
  <Element4>4</Element4>
  <Element5>5</Element5>
  <Child2>2</Child2>
  <Child3>3</Child3>
  <Child4>4</Child4>
  <Child5>5</Child5>
</Root>