I am thinking about clearing my beer with gelatin. Can I add the gelatin in the already finished and packaged beer in the keg?
As farmersteve said, yes you can.
But in my experience you can get less than desirarable results if done wrong.
Problem happens when force carbonation method or timing messes with the clarification process.
Best way is to add gelatin to the keg, swirl the keg slightly don't shake. Then use the top down plug and forget method to force carbonate. Top down Hi psi then low psi works too. Doing either of these tends to have the beer clear once done carbonating. The co2 seems to work like a piston driving down the gelatin and particles.
Doing bottom up or any aditation method doesn't do well. When doing this I found the keg to take just as long to clear as without gel. When cleaning keg I found most of the geltin in a ring of dense slime on top of the keg.
Adding gelatin to already carbonated beer works fine too. But don't disturb the keg and make sure it will maintain pressure. If it's able to lose carbonation it will float the gelatin.
Yes you can. It's done all the time. Article on Gelatin fining in keg