Softbox + Strobe + Bard Doors?

Barn doors are designed to be used on the strobe head itself, as is the softbox. They are not designed to be used at the same time. Even if you could jury rig it together, the light will spread as soon as it passes the edge of the doors and the softbox will diffuse it, rendering the barndoors pointless. If you need to flag off the softbox get some black fabric, duvatine is best, and lay it over the areas you do not want light to pass.