The B unit in your FP7 "set" is really an F7 B unit. Only the A units differed in length between FP7 and F7's. One option you have is to purchase a second powered FP7A unit and pull your train with ABA power. Another option is to purchase a powered F7B unit from the upcoming F7 production run and swap the chassis - selling off a cross kitted dummy B unit.
Ed Rappe