I'm using SS3.1, how can I replace the SecurityAdmin and MemberImportForm with my own custom methods to create a custom Member CSV import? I've tried

Object::useCustomClass('SecurityAdmin','CustomSecurityAdmin');
Object::useCustomClass('MemberImportForm','CustomMemberImportForm');

and

Object::add_extension('MemberImportForm','CustomMemberImportForm');
Object::add_extension('SecurityAdmin','CustomSecurityAdmin');

I've also attempted .yml settings, but I'm not very knowledgeable about YAML.

but neither of them work. Other than hacking the framework directly, is there a more elegant solution?

maybe this will work
MemberImportForm::add_extension('CustomMemberImportForm');

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.