There's no easy way to approach the subject,even those wives that would like the idea will be shocked at first by your suggesting such a thing.
I think the initial thought is that you can't still love them or do value them, it's a minefield.
I think the best approach is role play, tell her it's just a game to close her eyes and imagine your someone else.
Maybe push her to tell you who she was imagining in your place,let her see your cool with it, let her see it excites you also.
Play games out and about,asking her to point out men she'd choose to have sex with just for a laugh .
Another route is to tell her about friends of yours who've told you she was hot,and admitted they did fancy your wife.
Whatever the response,she will feel good knowing she's still got it.
Only you will know when the time is right to break the news that you'd let her have sex with a guy of her choosing ,you don't push it, you've put the idea in her head, and she will be thinking about it, just give her time .