My question is... Can we use Xcode to create a tweak for jailbreaked iPhone? Foe example if I want to add some button in the lock screen how can I do? How can I mod the GUI of the default iPhone applications?
This is a really old question, but David is actually incorrect on most counts.
You can add custom UI to jailbroken devices, create tweaks without Xcode, hook into anything you wish, and more, without even having the source code for SpringBoard or wishing for it to be extensible.
MobileSubstrate is a code replacement platform that literally allows you to do anything. You don't even need to be an expert, but admittedly, having at least a moderate grasp on programming and Objective-C concepts helps a lot.