Springs don't "get soft". They may, over time, shorten (sack-out) to a certain extent, but the rate does not change.
The only way to change a rate of a spring is to change the dimensions of the wire - wire length, diameter, etc.
While it may be possible to get a spring back to it's original length, I'm not sure there's any good - or safe - way to do it. When a spring is made properly, part of the process is to heat treat and set the spring. Springs that are done properly in the first place are the ones that don't break or sack out easily. If you heat the spring again to try and stretch it back to it's original length, you are messing with the original heat treat (if there was one) and the actual molecular structure of the material. This could make the metal brittle, causing the springs to break.
Don't waste your time trying to revive sacked-out springs. Spend the money on a good quality spring. That's the only way to get your suspension to work properly.