var tabpanel = {
    xtype: 'tabpanel',
    tabBar: {
        defaultType: 'xxx'
    }
}​

onAdd: function(item, index) {
    var me = this,
        cfg = item.tabConfig || {},
        defaultConfig = {
            xtype: 'tab',                // <= XXX
            card: item,
            disabled: item.disabled,
            closable: item.closable,

tabConfig: {
   xtype: 'xxx'
}