malsyst,
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<p id="myId"> Hello </p>
<div id="name_"> Block</div>
<script>
var a = document.getElementById('myId').id;
var blockDiv = document.getElementById('name_');
blockDiv.id = blockDiv.id + a;
</script>
</body>
</html>