You can't divide a country in two that simple. The CSA you propose would be an independant country in game, and would thus need a new tag... The easiest way to add a new country is still manually editing it in (but that's me, you can also use the editor).There are a number of free tags that editors can use, ranged from U00 to U19. The procedure is quite the same as for EU2.
1) Dive into text.csv and go to the line where U00 (for example) is defined. Change
U00;unknown to
U00;CSA.
2) Go into the USA scenario definition file (usa.inc) in the Scenario/36 (if you want to play CSA in 1936). Open province.csv simultenously. Then check which provinces CSA would own. Delete the corresponding numbers (to check those numbers use province.csv) from the usa.inc file BOTH under ownedprovinces and controlledprovinces
3) Create a new country definition file for your fresh country, name it u00.inc. For an example of how a .inc file should look, you can copy the content of the usa.inc file into the new file and edit from there. In the controlled & owned provinces lines, add the numbers of the provinces CSA should own
4) If desired, you can add flags, leaders, ministers, etc etc as well. Numbers 1 --> 3 are essential though. The rest is looks
