можно просто стилем
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>demo</title>
<script type='text/javascript' src='http://docs.sencha.com/ext-js/4-0/extjs/ext-all.js'></script>
<link rel="stylesheet" type="text/css" href="http://docs.sencha.com/ext-js/4-0/extjs/resources/css/ext-all.css">
<script>
Ext.onReady(function() {
var form = new Ext.form.FormPanel({
renderTo: document.body,
items: [
{xtype:'textfield', id:'text', fieldLabel:'', value : 'Поле которое надо центрировать', width: 200, style:'margin: 5px auto'},
{xtype:'numberfield', id:'year', fieldLabel:'Тест 1', value : ''},
{xtype:'numberfield', id:'month', fieldLabel:'Тест 2', value : '1'},
{xtype:'checkbox', id:'inv', fieldLabel:'Тест 3'}
]
});
});
</script>
</body>
</html>