My older cat absolutly hated it when we brung home our litte Pandora kitty home. He would meow and slap her and wouldn't let us mess with him for weeks! He eventually got over it and now he has him a new hunting buddy. They're the best of friends now.

Rabits are far more social than cats or dogs. They have an instinct to stick together for protection. I have never had a problem with an old rabbit not accepting a newer one, but you might want to do it slowly just to make sure. Put the newer one in a cage near the older one and switch out their bedding so they can get used to each other's smell. Let them out to play with each other after a while of this and see how it goes.