You achieve this by changing the control template of the FilterCell.

Here's an example:

<xcdg:FilterRow>

  <xcdg:FilterCell FieldName="IsBig">

    <xcdg:FilterCell.Template>

      <ControlTemplate TargetType="{x:Type xcdg:FilterCell}">

         <CheckBox HorizontalAlignment="Center"

            IsChecked="{Binding

            Path=Content,

            RelativeSource={RelativeSource

            AncestorType={x:Type xcdg:FilterCell}}}"/>

      </ControlTemplate>

    </xcdg:FilterCell.Template>

  </xcdg:FilterCell>

</xcdg:FilterRow>