Just use recursion:

private List<Node> allnodes = new List<Node>();

foreach (Node ne in ultraTree.Nodes)
{
    allnodes.Add(ne);
     if (ne.Nodes.Count > 0)
             WalkTree(allNodes, ne);
}

private void WalkTree(List<Node> allnodes, Node parent)
{
    foreach(Node n in parent.Nodes)
    {
        allNodes.Add(n);
        if (n.Nodes.Count > 0)
             WalkTree(allNodes, n);
    }
}

I didn't run this or anything but something like this should work, not sure if its the easiest way or not.  Or look up walking a tree in Google.

Is there an example somewhere on how to walk through the nodes?  Thanks.