DropDown items in Code

I'm using: PropertyGrid.AddNode("Programs", "", PropertyGrid.ListSelectEditorKey) to add a custom PropertyEditor however how do I add the list items in code?

This can't be done directly.

The simplest workaround is to use a custom template based on the built-in template.  The built-in template looks roughly like this (there's some extra stuff to handle type converters but you won't need that):

  <DataTemplate>
    <ComboBox SelectedItem="{Binding Path=Value, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, ValidatesOnExceptions=True}"
              ItemsSource="{Binding Property.Property, Converter={StaticResource EnumValuesConverter}}"
              IsEditable="True"
              BorderThickness="0" Margin="0" Padding="0" FocusVisualStyle="{x:Null}"
              Style="{Binding RelativeSource={RelativeSource AncestorType={x:Type ContentControl}}, Path=(ms:Editor.HostStyle)}" />
  </DataTemplate>

You can take a copy of this and replace the ItemsSource with your own ItemsSource.  Then load your custom DataTemplate using FindResource and pass that to AddNode.  (Be sure to cast it to a DataTemplate so AddNode doesn't try to interpret it as a key!)