Android Proguard

Using Proguard, I know that I can overshadow my code after the APK is generated, but can I perform this obfuscation before generating the APK? How to create a lib (.jar) and when does it matter it is already overshadowed? With this every time I make a change in my lib, and import it into a project it would be obfuscated, would it be possible to use this operation with Proguard?