Ive made small example on how to place RadGridView inside editable RadComboBox and filter the grid items as you type in the combo:

The easiest way to place any UI element in RadComboBox is to create single RadComboBoxItem and define desired Template:

<telerikInput:RadComboBox  Text="{Binding  Text,  Mode=TwoWay}"
                           IsEditable="True"  Height="25"  Width="200">
    <telerikInput:RadComboBox.Items>
        <telerikInput:RadComboBoxItem>
            <telerikInput:RadComboBoxItem.Template>
                <ControlTemplate>
                    <telerikGrid:RadGridView  x:Name="RadGridView1"  ShowGroupPanel="False"  CanUserFreezeColumns="False" 
                                              RowIndicatorVisibility="Collapsed"  IsReadOnly="True"
                                              IsFilteringAllowed="False"  ItemsSource="{Binding  Items}" 
                                              Width="200"  Height="150"  SelectedItem="{Binding  SelectedItem,  Mode=TwoWay}">
                    </telerikGrid:RadGridView>
                </ControlTemplate>
            </telerikInput:RadComboBoxItem.Template>
        </telerikInput:RadComboBoxItem>
    </telerikInput:RadComboBox.Items>
</telerikInput:RadComboBox>

Now you can create small view model and bind ...

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