Hi, 

I have a block which represents list of items:

    public class ShortListBlock : BlockBase
    {
        [Display(Name = "List Source")]
        [DefaultDragAndDropTarget]
        public virtual ContentArea ListSource { get; set; }

        [Display(Name = "List Display Options")]
        public virtual ListDisplayOptionsBlock ListDisplayOptions { get; set; }
    }

And I'd like to allow user drag and drop pages into the list source, tying something like:

    [EditorDescriptorRegistration(TargetType = typeof(ShortListBlock))]
    public class ShortListDroppableListSourceEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable attributes)
        {
            base.ModifyMetadata(metadata, attributes);
            metadata.AdditionalValues["DropTargetType"] = new[] { "epi.cms.pagereference" };
            metadata.AdditionalValues["DropTargetChildProperty"] = "ListSource";
        }
    }

But the default behavior replaces whole ContentArea contents with a dropped page. How to modify it not to replace but just add a new item into the ListSource content area?