It's possible to move from cheating to poly, but it's certainly extra hard, because chances are very good you'll be dealing with hurt, mistrust, and confusion. Still, you're getting ready to do the right thing now, and that's what matters.
I assume that either you love this woman, or the idea of poly really resonates with you, or both, for you to be willing to deceive your wife like this. Try to figure out which it is.
Do you love this new woman? If so, then the relationship, even though it came about via unfaithfulness to your wife, deserves some respect and you should tell your wife and perhaps also tell her you don't want to end it. If it's just about sex and friendship, on the other hand, I'd suggest stepping away from your relationship with the new woman. Then go to your wife and tell her everything, and that it's ended for the forseeable future. Admit your mistakes, beg forgiveness, *then* tell her that this woman introduced the idea of poly to you and it resonated, and ask if she would she be willing to read a little about it and possibly consider it.
Third possibility -- you don't love this woman and you don't really resonate with the idea of poly, but you and your wife are having other relationship problems that made an escape seem like a good idea?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.