When municipal systems soften water they often leave 6 to 8 grains of hardness per gallon in the water. That is not so easy to do with an ion exchange system that usually takes all of the hardness out of the water.
If the pressure drop across the softener doesn't vary over the time between regeneration cycles, it might be possible to put an adjustable bypass (a small valve) around the softener that would allow about 20% hard water to mix with 80% soft water. Two gallons at 30 grains + 8 gallons at 0 grains = 10 gallons at 6 grains.
There will be variation in pressure drop with flow, but that is ok because the pressure drop variation with flow will result in a similar variation in the bypass flow.
Here is an invention for you (patent pending ). You could run a cold hard water line to the cold side of a tempering valve. The mixed water will contain some hardness. The temperature settings could be figured out to result in the desired temperature and hardness.
A simpler solution would be to run the soft water only to the water heater. The water heater would be protected but the warm water would be all mixed some hard cold water. End of slippery showers.
Another variation would be to run the cold hard water line only to the shower.