Hello and welcome to the forum!
I understand the hesitancy of bringing it up because then the reality has to be dealt with. Your partner may be against it from the beginning and so you will need to make a decision. It could also ruin the friendship with the woman.
I'm fortunate because I started talking to my primary partner about it and within 3 or 4 months, he was resonating with the whole idea. I didn't insist or push it on him. I have been in sales for over 22 years and I do have a natural gift of being able to influence people, especially when it's something I am passionate about.
This is what I did.......First, I did my research and I processed things for myself. I came to a place where I simply knew that I am polyamorous and that I wanted to pursue a polyamorous lifestyle.
With that firm realization and the confidence to move forward, I began to share my thoughts and feelings, being vulnerable and authentic with all of the people I am close to- including my current primary partner (we were in a non-sexual friendship phase at the time but we had been sexually monogamous before).
Within 6 months, I had 3 male partners that I was seeing- Richard, John and Charles. But Charles got into a relationship with a woman who doesn't have the emotional maturity to be in a polyamorous relationship so I don't see him any more. And John decided he wasn't interested in polyamory at all, so that ended and I had to grieve that. But Richard and I are still together and we are on the same page about polyamory......so, it can happen!!!
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.