Code:
Ext.myPanel = function(config){
    this.newVar = 'i\'m new var!';
    this.newFunction = function(){ alert(this.newVar + ' and ' + this.newVar2); }
    
    Ext.myPanel.superclass.constructor.call(this, config);
}

Ext.extend(Ext.myPanel, Ext.Panel, {
    newVar2: 'i\'m new var 2!',    
    newFunction2: function(){ this.newFunction(); }
});


Ext.onReady(function(){
    var p = new Ext.myPanel({
          config
     });

    p.newFunction2();
});
alert shows "i'm new var! and i'm new var2!"

... both works fine