You will need to have your source collection interpreting the FilterDescriptors. Unfortunately RadDataFilter cannot control this. As your control does not expose FilterDescriptors itself, I can suggest you wrapping the source collection in such a proper collection. For example you can consider the QueryableCollectionView.
MyViewModel model =
QueryableCollectionView vvv =
Then use this instance as a source for your control. That way you can work with this collection and its FilterDescriptors collection.
How does this work for you?