Gere o evento de AfterSelect do treeview e escreva esse código:
private void treeView1_AfterSelect(object sender, TreeViewEventArgs e)
{
this.comboBox1.Items.Clear();
foreach (TreeNode node in e.Node.Nodes)
{
if (node.Nodes.Count == 0)
this.comboBox1.Items.Add(node.Text);
}
}