This example demonstrates how to configure GridViewComboBoxColumn setting ItemsSource property for it.
See also:
WPF: http://docs.telerik.com/devtools/wpf/controls/radgridview/columns/columntypes/column-types-combobox-column
SL: http://docs.telerik.com/devtools/silverlight/controls/radgridview/columns/columntypes/column-types-combobox-column