**** Update - Solved. Got it to work. There's nothing wrong with this approach.

For some reason, when I add the lines below, I cannot assign the datatable (zoekKlantSql.dt) to the combobox.datasource anymore.

        Dim insertRow As DataRow = zoekKlantSql.dt.NewRow()
        insertRow("KL_Nummer") = "0"
        insertRow("displayField") = "Insert"
        zoekKlantSql.dt.Rows.InsertAt(insertRow, 0)