OK, from reply with other post I managed to solve it fast and easy in code behind.
First, we have to get DocumentTabControl from DocumentContainer:
tabControl = VisualUtils.FindDescendant(sender as Visual, typeof(DocumentTabControl)) as DocumentTabControl;
Then we have access to every tab. So for my is just:
//find template and add it to first item and add it to first item:
DataTemplate template = TryFindResource(templateKeyName) as DataTemplate;
TabItemExt item = tabControl.Items[0] as TabItemExt;
if(item != null)
item.HeaderTemplate = template;