It would probably help to start with a definition of "soft focus". "Soft focus" is not just another way of saying "out of focus", at least not in the photographic vocabulary.

The classic soft focus lens is the Rodenstock Imagon. It was a lens that had a relatively large amount of spherical aberration, which means that the center of the lens doesn't focus at the same plane as its outside edges. Used without its signature "sink strainer" diaphragm, it was soft to the point that it was unusable. The diaphragms had a relatively large hole in the center, which, by itself, produced a relatively sharp picture -- not tack-sharp by today's standards by any means, but much better than the lens used without the diaphragm.

Around that center hole were a series of smaller holes that allowed light to pass through the periphery of the lens. Light from the periphery would be out-of-focus relative to the light from the center, so there was an in-focus image with an out-of-focus image overlaying it. (The size of the smaller openings was controllable, so you could adjust the degree of softness.) The Canon 135/2.8 SF lens uses a different method to achieve a similar effect, but it's the spherical aberration (the fact that the lens can be simultaneously in and out of focus) that does the trick. That means that simply blurring the image won't work -- at the very least, it needs to be a combination of a sharp image and one that's been blurred somewhat. More than one blurred image with different amounts/types of blur can work better.

Beyond that, you need to keep in mind that shadows (no light) from the out-of-focus part of the image can't contribute anything to the image. In the end, a classic soft focus image will be characterized by an overall loss of detail in bright areas, and a sort of etherial "halo" that bleeds from the highlight areas into the shadows. You can approximate that with a blending mode that only lighten the underlying sharp image, and that has a blend range that ignores the darkest tones in the blurred image (and preferably one that can graduate this cut-off).

Most of the higher-end image editors I've used can do this (with a greater or lesser degree of difficulty). Frankly, it's a heck of a lot easier to use a plugin. I know that Topaz Labs has a couple of plugins that will do it, and I'm pretty sure that NIK Color Efex can do it as well. No doubt Alien Skin and others can supply a plugin for Photoshop that will do the trick, and there are a number of plugins for the GIMP that do classic soft focus effects (I haven't evaluated any of them, so I can't recommend one to you).