Ok, now that that one was addressed let's get on to answering the OP
Your wife needs to feel trust IN you... not to judge, not to ridicule, and to accept what she says.
And yes, the Other post, apparently it WAS there, but, it was way out of our ballpark
How about trying a simple exercise and see how it works.. As foreplay, ask her to close her eyes and Imagine.. throw a scenario or two at her, Friends, Celebrities, Characters from TV or Movies... Get the ball rolling and then interject asking what they are doing now.. or what she is doing with them..
See how that goes... add in a few toys to simulate another person.. and what her reactions.. if she gets freaked out. STOP immediately. If she reacts positively, go slow and add more.. or more another time. Do not try fantasy play every time you get in bed, but use it to enhance your lovemaking, and if she gets the idea that Hey maybe it would be even better with another playmate.
Best of luck.