主题：如何增加列

Dim bd1 As New SQLGroupTableBuilder("统计表1","进货单")
Dim dt1 As fxDataSource
bd1.Groups.AddDef("型号") '根据型号分组
bd1.Totals.AddDef("数量","进货_数量") '对数量进行统计
bd1.Totals.AddExp("进货_金额","数量 * 单价") '对金额进行统计
dt1 = bd1.BuildDataSource()

Dim bd2 As New SQLGroupTableBuilder("统计表2","销售单")
Dim dt2 As fxDataSource
bd2.Groups.AddDef("型号") '根据型号分组
bd2.Totals.AddDef("数量","销售_数量") '对数量进行统计
bd2.Totals.AddExp("销售_金额","数量 * 售价") '对金额进行统计
dt2 = bd2.BuildDataSource()

Dim bd3 As New SQLGroupTableBuilder("统计表3","退货单")
Dim dt3 As fxDataSource
bd3.Groups.AddDef("型号") '根据型号分组
bd3.Totals.AddDef("数量","退货_数量") '对数量进行统计
bd3.Totals.AddExp("退货_金额","数量 * 单价") '对金额进行统计
dt3 = bd3.BuildDataSource()

dt1.Combine("型号",dt2,"型号") '将销售统计数据组合到进货统计数据
dt1.Combine("型号",dt3,"型号") '将退货统计数据组合到进货统计数据

With DataTables("统计表1").DataCols
    .Add("列名1",Gettype(Double), "IsNull([列名2],0)+ ISNULL([列名3],0)")
End With
dt1.Show("统计表1") '显示统计结果

上面红色代码中如何增加一表达式列？