I think there are a lot of blanks here. Do you actually want this relationship? Do you actually want to be poly? Are you responding to your husband's wishes? Do you want a girlfriend but possibly not this one?
I'm getting the feeling that you feel pressured into a relationship with a girl of your husband's choosing, so that he can sleep with the person he's attracted to. If this is an accurate assessment, then it sets off alarm bells. You should be choosing this for you, you should all be happy and comfortable, and this sounds like he wants permission to cheat. Plus--sheesh, no one can predict that you will "come to love" someone else, and frankly, in my mind that's a recipe for anger and resentment. I'm going to have to say that probably you should have a lot more communication and introspection before you move on.