Issue https://wpftoolkit.codeplex.com/workitem/20733 has been created.
In the meantime, if you have the "Plus" version of the Toolkit, you can create your own Properties with CustomPropertyItem. You can see it in the "LiveEplorer App" availale here : https://wpftoolkit.codeplex.com, in the sample : Data/PropertyGrid/Using a Custom List/Custom Properties. You can also have a look at this discussion : https://wpftoolkit.codeplex.com/discussions/463364
In the meantime, if you have the "Plus" version of the Toolkit, you can create your own Properties with CustomPropertyItem. You can see it in the "LiveEplorer App" availale here : https://wpftoolkit.codeplex.com, in the sample : Data/PropertyGrid/Using a Custom List/Custom Properties. You can also have a look at this discussion : https://wpftoolkit.codeplex.com/discussions/463364