How can I add post-flavoring to beer? I'm having a party and I want to add flavors to my already-brewed beer. How can I do that and have it mix?
Any time you mix things into beer you'll lose some carbonation. Any powder would nucleate bubbles very quickly, so the flavoring should be in liquid form. The liquid should be as similar to beer possible: cold, equally carbonated, and not too much alcohol if it can be avoided. Stir gently in a cold glass and serve right away.

Or, if you are brewing the beer yourself just add it at kegging or bottling time.