It's possible to move an existing relationship from monogamous to polyamorous when one partner doesn't like the idea but it's very hard. It's triply hard when you're coming from a place of cheating and broken trust. Possible, but it takes a LOT of hard work on everyone's part.
What's not possible is moving this woman into your home any time soon and keeping your marriage. How is your wife supposed to accept the presence in her home, the place that should be her safe space, of a woman who participated in deceiving her for 9 months? The living symbol of your infidelity and untrustworthiness? How is she supposed to cope?
Living together in a poly configuration is a daunting challenge when all the people involved like and trust each other. Even then, it's tough to negotiate issues of sharing, space, time, and jealousy. It takes a truly strong set of relationships. Attempting that obstacle course with a shattered foundation? It will blow up and blow up badly.
The only possible way forward that I see that has any chance for you to keep both relationships is for you to put the relationship with the au pair on hold, explain to your wife that you've done so and begin the process of making amends to her and exploring the idea of poly with her in a more sane way. Maybe, *maybe* you can get to the point with your wife where you and the au pair could continue something without ending your marriage, but not with her living with you any time remotely soon.
If the whole thing about her living with you is because she has no other way to be in the States, well... I hate to say it, but has it occurred to you that she may like you a whole lot but also have ulterior motives here? Also, has it occurred to you that, the cheating and the complete lack of realism aside, there might be other issues with elevating a physical affair with this barely-adult woman, who is literally half your age and who you only know as your employee, to the same level of importance as your 20 year relationship with your life partner?
I mean, where is this coming from? Trouble in the marriage, midlife crisis? No matter how many drinks you had on the night it started, a 9 month affair doesn't happen by accident.
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.