After that unfortunatelly there is not pure XAML solution. Instead you should do the following in codebehind.
get the resource as GridViewColumnCollection,
add the columns in the GridViewColumnCollection to RadGridView.Columns ( e.g. usng the RadGridView.Columns.AddRange method) .
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Thanks for the reply. It would be very useful though if one could define different column configurations as you demonstrated, then swap them in and out via triggers in XAML. It would then be easy to assign different datasources to the same RadGridView and custom tailor the columns in XAML.