Como adicino un item en blanco a un combobox

[aralar]

Esto es lo que tienes tú:
"SELECT sCustomerName, sCustomerID FROM tblCustomers order by sCustomerName ASC"

yo lo dejaría así:

"select null as scustomername, -1 as scustomerid from tblcustomers union

SELECT sCustomerName, sCustomerID FROM tblCustomers order by sCustomerName "

quedaría así, a ver si te vale.