You can do this, but you'll need to subclass a couple of things.

Start off with an extended stock item (and collection).  Here I'm just adding a transient field which calculates a return value from a standard field (Code) and a field in my AddIn table ("MyXTableField"):

   public class ExtendedStockItem : Sage.Accounting.Stock.StockItem
    {
        public const string FIELD_MyTransientField = "MyTransientField";
        protected Sage.ObjectStore.Field _MyTransientField;

        [Sage.ObjectStore.Builder.MetaDataField(FieldPropertyName = "_MyTransientField", DbType = System.Data.DbType.Int32,
            IsTransient = true)]
        public int MyTransientField
        {
            get
            {
                _MyTransientField.ValueRaw = this.Code.Length + this.Fields["MyXTableField"].GetString().Length;
                return _MyTransientField.GetInt32();
            }
        }
    }

    public class ExtendedStockItems : Sage.Accounting.Stock.StockItems
    {
        public override PersistentObject Owner
        {
            get
            {
                if(this.Query.Owner == null)
                {
                    this.Query.Owner = new ExtendedStockItem();
                }
                return this.Query.Owner;
            }
        }
    }

Now we'll extend StockItemLookup to add a new column and swap out the data source:

    public class MyStockItemLookup : Sage.MMS.Controls.StockItemLookup
    {
        private Sage.ObjectStore.Controls.ListColumn lengthColumn = null;

        protected override void Initialise()
        {
            base.Initialise();
            if (this.Columns.Count == 2)
            {
                this.lengthColumn = new Sage.ObjectStore.Controls.ListColumn()
                {
                    Caption = "Length",
                    AutoSize = true,
                    Sortable = false,
                    DisplayMember = "MyTransientField"
                };
                this.Columns.Add(lengthColumn);                
            }
            StockItems ds = this.DataSource as StockItems;
            ExtendedStockItems extendedItems = new ExtendedStockItems();
            extendedItems.Query.Filters = ds.Query.Filters;
            this.DataSource = extendedItems;

        }

From that point on it'll behave just like a StockItemLookup (because the Selected events will still treat the extended StockItem as its base type).

Hi Chris, it work. but I now I cannot use the StockItemLookup UI/Designer properties. Do I need to override these field as well?

Edit: Sorry my bad. this screen was for StockLookup. But doing it to StockItemLookup is okay. Now I will do the same to StockLookup