Nathan Berman is already working on the city’s largest office-to-residential conversion, and now he’s planning on topping it. Berman’s Metro Loft Management has struck a deal to convert the former Pfizer headquarters in Midtown to roughly 1,500 rental apartments, sources told The Real Deal. That would surpass the 1,300 units Berman is working to convert at 25 Water Street in the Financial District.