How to add Items with value and display into comboboxes?

Question

hi all i have a combo box that have a datatable dt as datasource

Code:

        dt = new DataTable();
        dt = DAL.ExecuteProc("SP_GetCashiers");
        CashierDDL.DataSource = dt;
        CashierDDL.DisplayMember = "Cashier_Name";
        CashierDDL.ValueMember = "Id";

just like that ,,note::CashierDDL is my combobox

i want to know how can add an item to my combobox with value to assign it to sqlParameter to send this parameter to another storedProc to get data based on seleted item from the combobox

i am working with vs2008 ,,winforms

thnx