Honestly, I think the first thing you need to do is talk about sex. An open, honest, frank and non-judgemental conversation, fully dressed and without erotic intent. Just a calm, open conversation about sex. Once you can do that, you can move on more in-depth topics like sexual desires and fantasies, which can reasonably lead to a conversation about swinging.
You can start as simply as "Dear, I'd like to talk to you about sex." Perhaps move toward: "I'd like to know what you enjoy/desire during sex." (Generally people love talking about themselves.)
Depending on your wife's personality and upbringing, you may get some push back, ranging from blushing to outright refusal. Don't try to force it. Be gentle, sincerely interested and, above all, set aside this ulterior motive of talking about swinging for now. Just honestly try to learn more about her inner sexual world and share yours with her. You may be surprised what you learn.