First, thank you for evaluating r.a.d.controls for WinForms. You're right, in design-mode you're provided with only several items which you can insert in a given RibbonBar chunk or button group. We've tried to keep the design mode experience simple and easy to use.
Anyway what you're trying to achieve is possible and you can use the following code:
RadCanvasViewport viewport = new RadCanvasViewport();
RadScrollViewer scrollViewer = new RadScrollViewer(viewport);
scrollViewer.AutoSizeMode = RadAutoSizeMode.FitToAvailableSize;
scrollViewer.MinSize = new Size(70, 60);
RadButtonElement testButton = new RadButtonElement("Button");
In the sample code above we've added only one button in the scroll viewer but you can add more buttons in a StripLayoutPanel for example and put it inside the Viewport.
I hope this information helps.