I would start by talking about it while you are fucking. The more arroused the better. She will be more receptive. I convince my wife to get a sensual massage. She was very hesitant but loved it. One guy we saw a few times would eat her after the massage. With another guy I suggested she suck him as I was fucking her on the table. She dove right in and let him cum in her mouth. The next time it was a full blown threesome which she loved. She was scared when his dick first touched her pussy but literally screamed when he came in her.