Hi Christoph,
Thanks for your interest in Syncfusion products,
To add a form or control as a tabbed document you can use the following code,
this.dockingManager1.SetAsMDIChild(form, true);
Sample demonstrating the usage of DockingManager with TabbedMDIManager is available at the following link,
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=DockingMAnager1678956048.zipYou can fine more information on MDI Child Transition at the following link,
Essential_Tools_Windows_User_GuideUnder Tools Controls >> Docking Package >> Concepts and Features >> Advanced Features >> MDI Child Transition
Please let us know if we have misunderstood your query,
Regards
Vallarasu S.