You can become poly through cheats (Options -> Difficulty Options -> Enable cheats, then Character -> Character Inclinations -> Type in "Polyamorous" and press Grant)
Or you can date the BFF (follow the quad storyline to its current conclusion, or take a shower in the morning to possibly get a scene that will start an alternative route to romance the BFF then keep talking about sex until you get another scene, and you will then get a "what are we?" talk with the BFF where you can choose to be poly.)