You can always add a soft filter to your camera for the effect. This is non-reversible. There is no way you can some how "unsoften" the image taken with a soft filter.
Soft filters (softeners) from differnet brands creat different results. Which ever suits your taste is, well, your personal choice. My personal favourite (before I learnt digital processing) is Nikon's, but I have only used Hoya and Nikon before. Now I do not use softeners and only rely on PS for such effects.
There are also DIY methods, such applying a thin layer of Vaseline on a skylight filter. I have not tried any of those. Also, DIY methods may not gove you consistant results.
Never heard of softening images during film processing or printing process. I suppose a softener can be added to enlarger type of printing in the darkroom.
Not sure also if softening effect can be added by the lab if you send a digital file for print. Ckiang should know a lot more about lab printing.
I remember long time ago I developed a roll of film a few months after I have taken the pictures, and the prints came out a bit soft.....