Same procedure as when creating custom brushes from selections - feather it. Read up in the help. Otherwise I'm not sure what you are getting at. There are literally trillions of brush collections out there and it should be easy enough to find soft brushes in all flavors.
Like Mylenium says, none of the sampled (non symetrical) brush shapes allow hardness to changed on the fly. If it is really important to you, stamp the brush in black on an empty layer; feather it, and redfine the brush as a new brush. But it makes way more sence to simply add Gaussian blur after laying down the brush stroke.
I do custom retouching on supplied images for B2B clients, ad agencies. Often times I'm blending in areas of differing focus or softness. The round brush allows changing the hardness 0-100% on the fly, which I use 90% of the time. I was using the spatter and chalk brushes to simulate an un-even surface. But I need to vary the hardness of the stroke as I work in the different areas of sharpness. I started back in Photoshop 1, currently on CS6, so over time I was hoping that maybe someone had developed a work around for varying the hardness of the other brushes or any defined brush for that matter. (or photoshop itself had a way that I wasn't aware of).
Using a gau-blur after each stroke is a slow process. Stamping in black on an empty layer and feathering it is another option. Still slow, as I'd need to create a half dozen different hardness brushes. But it is a solution, unless someone has a better way.
thanks for the input.
Using a gau-blur after each stroke is a slow process.
Sounds like you know the program better than some of the regulars here. Best I can think of off the top of my head, is to use Gausian blur at a level slightly less than needed fpr the brush sizes you typically work with, then asign a single Function key to an Action that either does a Cmd f (You have to be a Mac use to go back that far) to repeat the last filter, so you could build with repeated hits of that Function key. But that is less than ideal, because it operates on the whole layer.
I tried to make Fade work to soften the last brush stroke, but I couldn't make it do anything other than Opacity, and none of the blend options help.
How about asking in the Scripting forum? There some really clever people over there who might at least provide a means to blur just the last stroke and not the entire layer. It's an interesting problem though, and I'll keep on thinking about it.
Check out some of the Wacom airbrush presets. Some of them do have a 'hardness' slider, but I don't think it's what you want. But you can adjust spatter size and amount, plus granularity, to get a lot of control of the texture you lay down, and get more control with brush pressure. We are all having to second guess exactly what look you are trying to achieve. Perhaps you could upload a non client example?
[EDIT] I should have added to have the brush panel open and the Tip Shape tab selected.