Extending HTML5 components

Is it possible to extend the list of web components? If so, how can we add a new library of components?

As an example, I wonder if the Telerik components can be added.

Yes. You can use the tool “Insert Component” to add an external HTML5 Control. Select the option HTML5 control of “Insert Component” to do that. Double click on HTML5 control inserted to open a dialog to insert the CSS Script and JS Script.