If she is already into swinging than the jump to interracial will be a small one as it really is the same thing. If your wife is not into swinging than start by just having an honest conversation with her. She might be up for it, but don't be surprised if she is not. Some women are just going to be against it no matter what and if you push to hard she is going to think your weird.
When you have this conversation do it at the right time!!! Not when the kids are screaming or in the supermarket...... are you listening men? Do it when she is already excited and a little drunk would not hurt.