Hi,
I am afraid, the office 12 rendering is not possible with MainFrameBarManager, since barManager does not have a Renderer property for applying the office12 rendering.
We will consider this as a feature request # .Please let me know if you have any questions.
Feature Request #626 - Add support to Office12 rendering for XPMenus.
Thank you for using Syncfusion products.
Regards,
Mano