In you need to change the FontFamily list of the settings pane, you can change the ItemsSource of ComboBox from which the users can choose a FontFamily. In order to achieve this you need to make this change in the Template of SettingsPaneTextControl. For your convenience I implement this approach in the attached project (I have used NoXaml binaries and VisualStudio theme). Please take a look at it and let me know if this works for your scenario.
Please have in mind that this will affect only the SettingsPane. If you need to change the FontFamily of all shapes in diagram, you can set this property in a style targeting RadDiagramShape.
Telerik by Progress
Telerik UI for WPF is ready for Visual Studio 2017 RC! Learn more.