I was able to do it simply using XPathNavigator in C #. In this case I'm putting nf along with a general notes.xml file where all the invoices are.
private void button2_Click(object sender, EventArgs e)
{
XmlDocument document = new XmlDocument();
document.Load("nf1.xml");
XPathNavigator navigator = document.CreateNavigator();
navigator.MoveToChild("nfeProc", "http://www.portalfiscal.inf.br/nfe");
XPathDocument newBooks = new XPathDocument("C:\Program Files\VertrigoServ\www\Prototipo\notasGeral.xml");
XPathNavigator newBooksNavigator = newBooks.CreateNavigator();
newBooksNavigator.MoveToChild("nfeProc", "");
foreach (XPathNavigator nav in newBooksNavigator.SelectDescendants("NFe", "http://www.portalfiscal.inf.br/nfe", false))
{
navigator.AppendChild(nav);
}
document.Save("C:\Program Files\VertrigoServ\www\Prototipo\notasGeral.xml");
label1.Text = ("Concluído!");
textBox1.Clear();
}