[underW]

ну, глянь это:

Ext.onReady(function(){


function ShowWindow(){ new Ext.Window({

title	: "Window",
height	: 400,
width	: 400,
items	: new Ext.FormPanel({
id	: 'form',
        labelWidth: 75, 
        frame:true,
        title: 'Simple Form',
        bodyStyle:'padding:5px 5px 0',
        width: 350,
        defaults: {width: 230},
        defaultType: 'textfield',
        items: [{
                fieldLabel: 'First Name',
                name: 'first'
            }],
        buttons: [{
            text: 'Добавь поле',
            handler: function(btn)
            				{btn.findParentByType('form').add({
            							xtype: 'textfield',
 						                fieldLabel: 'First Name',
                						name: 'test'
            });Ext.getCmp('form').doLayout();
        }
       
            }]
    })
,
closeAction: 'close'


}).show()
}

new Ext.Button(
{
text	: "Покажи окно",
handler : function(){ShowWindow()},
renderTo : Ext.getBody()
}
)
})