<Snippets>
  <Snippet name="abc">
    <SnippetCode>
      testcode1
    </SnippetCode>
  </Snippet>

  <Snippet name="xyz">
    <SnippetCode>
     testcode2
    </SnippetCode>
  </Snippet>

  ...

</Snippets>

XDocument doc = XDocument.Load(spath);
        XElement root = new XElement("Snippet");
        root.Add(new XElement("name", "name goes here"));
        root.Add(new XElement("SnippetCode", "SnippetCode"));
        doc.Element("Snippets").Add(root);
        doc.Save(spath);

<Snippet>
    <name>name goes here</name>
    <SnippetCode>
    code goes here
    </SnippetCode>
  </Snippet>

<Snippet name="abc">

XDocument doc = XDocument.Load(spath);
 XElement root = new XElement("Snippet");
 root.Add(new XAttribute("name", "name goes here"));
 root.Add(new XElement("SnippetCode", "SnippetCode"));
 doc.Element("Snippets").Add(root);
 doc.Save(spath);

XElement root = new XElement("Snippet");
root.Add(new XAttribute("name", "name goes here"));
root.Add(new XElement("SnippetCode", "SnippetCode"));

<Snippet name="name goes here">
  <SnippetCode>SnippetCode</SnippetCode>
</Snippet>

public static void Test()
        {
            var xdoc = XDocument.Parse(@"
<Snippets>
  <Snippet name='abc'>
    <SnippetCode>
      testcode1
    </SnippetCode>
  </Snippet>

  <Snippet name='xyz'>
    <SnippetCode>
     testcode2
    </SnippetCode>
  </Snippet>

</Snippets>");

            xdoc.Root.Add(
                new XElement("Snippet",
                    new XAttribute("name", "name goes here"),
                    new XElement("SnippetCode", "SnippetCode"))
                );
            xdoc.Save(@"C:TEMPFOO.XML");
        }

<?xml version="1.0" encoding="utf-8"?>
<Snippets>
  <Snippet name="abc">
    <SnippetCode>
      testcode1
    </SnippetCode>
  </Snippet>
  <Snippet name="xyz">
    <SnippetCode>
     testcode2
    </SnippetCode>
  </Snippet>
  <Snippet name="name goes here">
    <SnippetCode>SnippetCode</SnippetCode>
  </Snippet>
</Snippets>

var child = new XElement(parentElement.GetDefaultNamespace()+"Snippet", new XAttribute("Attr1", "42"), new XAttribute("Attr2", "22"));
child.Add(new XAttribute("Attr3", "777"));
parentElement.Add(child);

var child = new XElement(parentElement.GetNamespaceOfPrefix("namespacePrefixThatGoesWithColon")+"Snippet", new XAttribute("Attr1", "42"), new XAttribute("Attr2", "22"));

<Snippet name="abc">

root.SetAttribute("name", "name goes here");