You can create a module and override file ( app/code/core/Mage/Adminhtml/controllers/Sales/OrderController.php ):
There you will find function addCommentAction:
https://github.com/MrRio/magento/blob/master/app/code/core/Mage/Adminhtml/controllers/Sales/OrderController.php#L191
and / change line of code from :
$order->addStatusToHistory($data['status'], $data['comment'], $notify);
To :
$current_Admin = Mage::getSingleton('admin/session')->getUser();
if(isset($current_Admin)) {
$commentwithUser = '[ BY: '.$current_Admin->getUsername().' ] - '.$data['comment'];
}
$order->addStatusToHistory($data['status'], $commentwithUser, $notify);