Using non request variable in validation.

Can I create new variable inside controller that will be used in validation placeholder?

PHP Code:

$id = 1; //this way or by adding request variable
$_POST['id'] = 1; //this doesn't seems to work.
$validation->setRules([
    'email' => 'required|valid_email|is_unique[users.email,id,{id}]'
]); 


I can't figure it out and also wasn't able to find any answers.