You cannot push or pull someone into the swinging lifestyle without risking your marriage.
Talk about the idea, but don't push it. If getting involved is not something you both want to do, don't do it - it will end in tears!
You can try to awaken their interest in Swinging by showing them articles about swinging, showing them ads from a cute girl/guy who may turn them on.
Try talking about fantasies whilst making love, if your partner responds positivly to the fantasies show them ads from people who could make the fantasy come true.