You can accomplish this, with defining your own custom logic that will update (either through bindings, or manually) the business object properties since this cannot be integrated in the default Edditing mechanism of RadGridView.
You can open the Popup on MouseLeftButtonDown/Up for RadGridView. You can get the cell that has been the source of the mouse event by using the e.OriginalSource property.
Hope this helps.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.