You need to look at the TabControl items collection for the tabitems
You can add another and set its content, e.g. if I have a tabcontrol with 1 tab page, on that tab page a button, in the click event you could;
private void button1_Click(object sender, RoutedEventArgs e)
{
TabItem newTab = new TabItem();
newTab.Header = "Another Tab";
newTab.Content = new TextBox();
tabControl1.Items.Add(newTab);
}