"The softness of the light is directly proportional to the relative size of the aperture of the light to the subject." The aperture of the light is the size, for example a 4' x 6', and has nothing to do with the lens aperture.
A Kino Flo has a larger aperture than a naked Fresnel or open-faced lamp, so it's softer at any given distance from the subject. That's the "yes" part of the answer.
On the other hand, the Kino Flo aperture is much smaller than the size of most softboxes; that's the "no" part of the answer.
So, here's the deal; Kino Flo fixtures with out diffusion to increase the size of the aperture are soft, but not as soft as a larger aperture softbox.
I would never put a Kino in a softbox, however; their silk is too dense, designed for hot, higher wattage lights. Just put so light diffusion over the Kino Flo doors and that will do the trick.
I like very light diffusion for many shots because it has a bit of hot spot, giving a nice soft look with more snap than a solid softbox.