Posted 12 June 2012 - 05:39 AM
This already takes place as all user's are given an ID (user_id) at the database level in the table cscart_users.
Depending on what you want to do with this you can query against it, display it back in the admin, or use it in an addon / extension pretty easily as it's pulled up with order records.
Just keep in mind that this is an auto-incrementing ID meaning that it will give away how many customer accounts you have as it keeps going up by one. If you're not willing to share that you'll need to hack in another field or change how this one is generated.