CellStyle BasedOn default CellStyle

I made a custom control called CustomDataGrid that inherits from DataGrid, and I put it in a DLL. Overriding the metadata I set for this control a new default style, different from the standard DataGrid one: this style include a custom CellStyle.

<Style TargetType="{x:Type prj:CustomDataGrid}">
...
        <Setter Property="CellStyle" Value="{StaticResource RightAlignCell}" />
...
</Style>

I use this DLL in a WPF project and I need to modify the CellStyle, but I can not reach the {StaticResource RightAlignCell} resource (located in the DLL) from the application. In other words I need to do something like:

<Custom:CustomDataGrid>
    <Custom:CustomDataGrid.CellStyle>
        <Style BasedOn="{StaticResource RightAlignCell}">
            <Setter Property="SomeProperty" Value="SomeValue"/>
        </Style>
    </Custom:CustomDataGrid.CellStyle>
</Custom:CustomDataGrid>

How can I perform it?
Thanks in advance