I think coming straight out with it would be like a hammer if you know she's never been that adventurous. It could mean a lot to her in the way of questions about your relationship and if it's in trouble. Personally I'm not into that and for me it could translate into questioning what we have together and why my partner needs this and not just me to get off? Not to say that it's wrong or doesn't work for some. Not knocking it. But remember, whatever butters both of your breads.
I recommend you bring it up within some teasing chatter about sexual fantasies. Maybe lead in with toys she may want to try, roll playing and such. Then kind of throw it in. Ask her if she ever thought about swinging? I just would avoid dropping a bomb about it. You know her better than any of us could. If she was open about telling you about having a sexual dream about another guy is that a hint? Is she the one in fact inferring? Whatever the case, mix it into a conversation about other sexual things that way you can always back out gracefully without it being the elephant in the room from that point on in your relationship.