That doesn't work either. MemberImportForm does loads of initializing in the __construct(...). The constructor can't be called when extending Extension rather than Form.
Tried going one level deeper to SecurityAdmin, but similiar issue with calling parent::init() function. Fatal error: Call to undefined method Extension::init() in E:\localhost\cbyc\mysite\code\CustomSecurityAdmin.php on line 35. I get stuck as I don't know how I can call the parent of $this->owner.