I do this for a living, and no, adding salt directly to the water will not help, as stated above. In fact, you will only be adding to the total impurities in the water (your total TDS and TNDS {total disolved, and total non-disolved solids})

However, using a soft water system as described above will take you some time, and use quite a bit of salt. I am fortunate enough to have 2 systems, and based on my hardness coming in (about 15 gpg) I have about 2200 gal capacity maximum- filling my 7000 gal pool, I used 60 lb of salt... each area will be different depending on how your city supply is, and where it comes from, here in FLA , it is ground water, usually very very hard.

The ratings are based on how much salt is used during the regeneration cycle, based on that, the water will flow to 0 gpg, if that amount of salt was not used, your water will be soft-er, but you will only be getting 80% and less the more water you run through the machine.
(i.e. 35000 grain unit {which is a inacurate number, check with WQA for all of that data, only a few units out there that will do that amount, acuratly} anyway, 35000 grain unit needs about 15 lb of salt to regenerate, to give it a 35000 grain capacity, or what we refer to as start capacity. Most units sold are set to 50% or less, to prevent the media (resin, or resin/carbon, or other combinations) from deteriorating too quickly- and use only about 5 to 8 lb of salt, giving them an actual 17000 grain capacity...


the above method will work, it will just take some time. AND, do make sure you don't let the backwash of the softener drain on your grass, it WILL kill it most of the time! (brine rinse is usually at total salt to water saturation)

anwyay, hope this helps in some way, or at least educates a bit on water treatment... I highly suggest to anyone else reading this, if they have vinyl, or fiberglass, to fill entirely with soft or treated water (0 hardness) the water feels incredible!