**** 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)
If things have the tendency to go your way, do not worry. It won't last. Jules Renard.