You can paste HTML code in the RadEditor form any where in you code as long as you have a reference to the control. You can use the PasteHtml
client side method of the RadEditor. There is more information on the matter in the following help article http://www.telerik.com/help/aspnet/editor/pastehtml.html
|if (typeof(RadEditorNamespace) != "undefined")
| //enter your code here.
I hope this helps.
the Telerik team