I would think that the reason that Critical Defense, Critical Duty, Pow'r Ball, et el. bullets work is because the elastomer/soft plastic plug in the nose of the bullet - when moving at 1100 fps - is "fluid" when it hits something creating a hydrostatic shock inside the cavity that starts the expansion.
A steel BB will definitely not be fluid at 40s&w velocities and will more then likely just act like a FMJ unless the BB falls out.
Also, you might be approaching the definition of "armor penetrating" hand gun ammo (GASP!) by adding the steel to it.
You might consider trying filling the cavities with readily available elastomers like Right Stuff Gasket Maker.