add sub district in the shipping address

I would need to know how to add a combo box in the shipping address sheet in order to add more geographic information(district, open cart doesnt have that for my country and i need it). I know I would need to modify the data base but all information regarding this would be appreciated.

basically i need to create sub zones just for my contry is that posible ??

i also need to add the sub districts, if any one know, pls help

Thanks in Advance