I want to add multiline item to ribbon menu (Microsoft.Office.Tools.Ribbon.RibbonMenu) in my office add-in like following image. but I don't know what these items should be?
*RibbonButton doesn't support multiline Label.
*RibbonLabel doesn't support variable font styles (like Bold and Normal).
UPDATE: if these features are not supported, is there another component (say DevComponents, Telerik, ...) that supports them?
Unfortunately not all ribbon features are available for developers. There is no way for add-in developers to implement something like that unfortunately.
You can read more about available ribbon controls and their attributes in the following series of articles in MSDN: