One thing missing, which is "[]" at the end of the variable name where you want to insert the new array:
We have:
$regras = array(
array(
'field' => 'cpf',
'label' => 'CPF',
'rules' => 'required'
),
array(
'field' => 'senha',
'label' => 'SENHA',
'rules' => 'required|trim'
),
);
To add another array inside this array:
$regras[] = array(
'field' => 'nome',
'label' => 'NOME',
'rules' => 'required|trim'
);
DEMONSTRATION
In fact you even have more methods:
array_unshift , add to the beginning of the array:
$add = array(
'field' => 'nome',
'label' => 'NOME',
'rules' => 'required|trim'
);
array_unshift($regras, $add);
array_push , you add at the end (similar to the first example I gave):
$add = array(
'field' => 'nome',
'label' => 'NOME',
'rules' => 'required|trim'
);
array_push($regras, $add);