K Cl salt or Potassium Chloride will work fine in the softener, it may have a little lower performance in regeneration, just use a little more.
Also the Potassium is good for the grass, it will not kill it, it is good fertilizer, also good to stop muscle cramps!!
Also, if you put the regeneration water in the sewer and a septic tank is used the Sodium may cause problems in the Septic Tank.
Potassium should cause no problems in a Septic Tank.