|"Cannot add items to the combobox when a datasource is assigned"
You need to add the blank row to a datatable before you bind it to the combobox.
This is off the top of my head so will need debugged obviously but you will get the idea:
SqlConnection MyConnection = new SqlConnection(PublicVars.ConnectionString);
SqlDataAdapter sqlDA1 = new SqlDataAdapter("SELECT ItemID FROM some_items", MyConnection);
DataSet sqlDS = new DataSet();
DataTable tblItems= sqlDS.Tables["some_items"];
DataRow NewRow = tblItems.NewRow();
NewRow = "-- Click To Select --";
cboItems.DataSource = tblItems;
cboItems.DisplayMember = "ItemID";
cboItems.ValueMember = "ItemID";
cboItems.SelectedIndex = 0;
You should get the idea.