Sooo, after some testing at home here is a Working Code:
FluxCP / modules / account / create.php
-> http://pastebin.com/puvFgRkN
FluxCP / themes / default / account / create.php
-> http://pastebin.com/ujLTZCnG
FluxCP / lib / Flux / LoginServer.php
-> http://pastebin.com/ekjVx9TH
You need to create a new coulum at your "login" table that is called "new_field"!
Replace all Files with the new Code from pastbin.com
I hope i can help you with it.