Something like this might work:

    [EditorDescriptorRegistration(
        EditorDescriptorBehavior = EditorDescriptorBehavior.ExtendBase,
        TargetType = typeof(YourProperyType))]
    public class YourEditorDescriptor : EditorDescriptor
    {
        public YourEditorDescriptor()
        {
            // Here you can set you editing class and so on
        }
    }

well, I wanted to use one of the built-in editor descriptors for my custom class, but looks like it works to derive my own editor descriptor from the Epi class and specify my custom type in the registration -- Thanks.