The default notification bar in Nexus S is very basic. It doesn't have any toggle buttons like others do. Can I replace it?
From what I've seen this is not generally very easy to do. I know that replacing the icons and such requires modifying the underlying
framework-res.apk file, and I've never really seen anything that modifies the functionality of the bar itself (aside from custom ROMs with custom frameworks and the like). I'm not completely positive on this, however.
If you just want to get the toggles, though, you could try an app like Notification Toggle. It says that it's a little bit hit-or-miss depending on the device, but I'd imagine that the Nexus S would be on the better end of the scale.
If you are just looking for toggle buttons in the notification bar I would check out Widgetsoid, it's a very full-featured app that will let you create widgets or notification toggles of whatever you want.
If you are looking to change the way the notification bar looks, you will need to have root access and install a theme, which would void your warranty and is only for the highly technical user.
If you want to change the status bars colors and icons, this video shows you how by modifying framework.apk. This video was done on a windows operating thus the particular method used may not apply to all operating systems.
Alternatives is downloading Super Status Bar or Status Bar+ from the Play Market. They are both free but also have a paid version. For the toggle in the notification bar you wanted, heres Notification Toggle, and heres another free toggle buttons app called Toggle Settings.