You can use the DocumentFragment elements and the InsertFragment method of RadRichTextBox.
DocumentFragments can be created from a RadDocument in the way specified in this forum thread. (A RadDocument can be created either programmatically or through import.)
After creating the DocumentFragment, you can insert it using the InsertFragment method of RadRichTextBox. In this way, the fragment will be inserted at the caret position, which you can adjust using the methods of DocumentPosition.
I hope this helps.
the Telerik team
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