Hi Pedro,
It depends on what that library does. If it's a JavaScript library, you can just add it to your app and use it, both online and offline. If it's a Cordova Plugin, you need to create a wrapper for it so it can be used by your app. This also can be used both online and offline (unless the plugin needs an online connection of course).
The reasons for extensions only being available for server-side logic is because they are written in C# and .NET, which are server-side only technologies.