Add value in sequence to the end of the value of a php array

I would like to add values in sequence to the end of each value in my array. So that when the loop is executed add values to the end of my array. Example in pseudo code:

// Array original
$arr = ['1' => '1.','2'=>'2.'];
FUNÇÃO QUE ADICIONA VALOR EM SEQUENCIA ATÉ 2 (for($i = 1;$i < 2; $i++))
$arrComValoresAdicionados = ['1' => '1.','2'=>'2.','3'=>'1.1','4'=>'1.2','5'=>'2.1','6'=>'2.2']