I don't know if this experience will help but here it is:
Before I ever became interested and more exposed and open minded to the idea of polyamorous relationships, a good looking man with approached me at a party (friend of a friend). I was happy to spend the night talking and dancing with him. He was very charming and good looking. Finally, near the end of the night, he told me that he had a girlfriend but that it was ok because he was in an open relationship and he wanted to kiss me. I gave him the cold shoulder right away and felt that he was a sleaze ball. He showed me a picture of his girlfriend on the phone and tried to explain that it was ok with her if he saw other women. I completely didn't understand and asked him why. His answer was, "so i can kiss pretty girls like you". The visceral response i had was completely negative. THough i appreciated his honesty i felt fooled. I didnt feel that our connection was special anymore and assumed he wanted to 'use me for sex' (which is possible lol).
Anyways, the point is, if the same person had done the same with me today (like 5 years later), i probably would ask more questions about his intentions and be more open to it. But since it was completely new to me i took the whole thing as an insult and got defensive and lost trust in him because i didnt get it.
If you are really interested in this girl, i would be cautious. even some of my own girlfriends still think I am psychologically unhealthy for being open to polyamorous relationships. Some people have strong feelings against it in the context of their current views, experience and exposure. I would agree forming some sort of trust and also learning more about her relationship experience and views may help you better determine how to approach the subject. If how you put the moves on her is purely sexual by what you say and do on top of already having a girlfriend, she may get insulted or very suspicious. focus on communicating something genuine.
Love is limitless...