コンボボックス型セルのShowButtonプロパティをTrueに設定します。
実行例：
ComboBoxCellType cbCell = new ComboBoxCellType() { Items = new string[] { "要素1", "要素2", "要素3" }, ShowButton = true, }; FpSpread1.Cells[0, 1].CellType = cbCell;
Dim cbCell As New ComboBoxCellType() With { .Items = New String() {"要素1", "要素2", "要素3"}, .ShowButton = True } FpSpread1.Cells(0, 1).CellType = cbCell