Add County box to customer details

I am getting ready to add the local sales tax mod. However, one field I will need it to calculate on is county. For a buyer in Alabama I have the correct sales tax. However, I need a way to put on new customer registration that if they are in the state of Alabama have them choose county. This way if they are in Lee county I can charge the additional county tax. All other counties would be charged just the state tax. From what I read the local tax mod will do this based on a county field. I just need to know how to add the county field. Thanks for any help on this.

https://www.jaylyns.com

Here is a thread that might be valuable:

http://www.zen-cart.com/forum/showthread.php?t=93099&highlight=add+county+box+to+customer+details

that thread didn't really apply to what I am trying to do. In the thread they choose county in the UK, similar to choosing a state in the US. I need the customer to be able to choose country(US), State (Alabama), and then if state is Alabama I need them to be able to select which county(lee Co or other). Any other ideas would be greatly appreciated.