open modal form to add new record on select new from dropdow

i have a sales.aspx web page that accepts sale info, such as customerid,productid,and qty.

 

actually a dropdown is used to display list of customer.

where as if the customer is new i first need to go Add_Customer.aspx  and add customer and select it later here on sales .

 

is there any provision to have a list item called NEW  in customerdropdown control to open an Add_Customer.aspx  and add new customer and that new customer is seleted in sales.aspx web form .

 

if anyother method is effiecient then wel.come