The default property editors will populate the ComboBox with the "standard values" of the property. The standard values are defined by the associated TypeConverter. If you are using SelectedObject(s), the TypeConverter can be defined on a Type directly, or on individual properties. The first post in this forum post
shows an example of a custom TypeConverter
If you are adding properties using Properties/Items/ItemsSource, then you can only define the TypeConverter on the type itself.
Some other options include:
1. Build a custom DataFactory and returning custom IPropertyDataAccessor objects, which in turn have a StandardValues collection.
2. Build a custom property editor. This is just a DataTemplate that is used to present one or more properties. In this case you can include a ComboBox with any items you want.