The requested feature is a bit out-of-the scope of RadControls for ASP.NET AJAX controls because they are UI controls which the developer can integrate in web sites, but not full blown CMS system as Sitefinity which you can use to create web sites.
This functionality is partially supported by RadEditor for ASP.NET AJAX, which offers an Insert Form Element dropdown and Node Inspector module for setting the properties of the form elements: http://screencast.com/t/Bkjto1JwPRm8
All the best,
the Telerik team
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