The built-in GridDropDownColumn is designed to be used mainly with DataTables in order to easily map and generate their dropdown items values in conjunction with the DataField/ListValueField/DataSourceID/ListDataMember
values of the column.
For custom objects collections or when you want to change the source of the dropdown editor dynamically, consider replacing the GridDropDownColumn with template column holding MS DropDownList in its edit template. This will allow to generating/binding the items for the MS DropDownList inside the ItemDataBound handler of RadGrid or change the DataTextField/DataValueField/DataSourceID of the dropdown editor.
the Telerik team