Actually, it is a bit beyond the functionality of the control to throw exceptions about its underlying collections and business objects (in case it is for validation). The ability to insert and remove items is not directly provided by the control - it actually counts on the source and the business object for doing so. Will it be possible to test the case where you cannot add new item with another control - RadGridView, DataGrid, ListBox, etc. in order to verify whether the problem is with RadDataForm and not with the source ?
the Telerik team