Low Calcium Hardness in pool water means that there is low calcium in the water and is considered "soft" and soft water tends to be corrosive. It will etch away and your plaster and any metal object the pool water touches.
Hardness increaser from your pool supply company
If the calcium is a result of pool chemicals, draining some or all of the pool water will lower the calcium hardness level. If the cause is the fill water, commercial hardness reducers or chelating agents will bond with the calcium to keep it trapped in solution.
I would only connect the soft water to the auto-fill if you are having a problem with the pool water being 'too hard'. If you do not have hard water in the pool, there is no advantage in using soft-water for topup. You can use the soft water BUT you MUST maintain the correct level of calcium hardness in the pool water (200-275ppm is good). If the calcium hardness falls too low other problems can occur, including damage to plaster walls.
Calcium chloride increases the hardness of water. Proper hardness is critical to balancing the water in a pool. Proper hardness is especially important in a plaster pool to prevent leaching of the plaster finish, which results in a rough surface. Calcium chloride can be purchased at home improvement stores at significant savings over pool supply outlets.
Your incoming fill water may be hard.
You can, if the water presently in the pool is 'hard' - but be very careful to keep an eye on the Calcium Hardness of the pool water. If you filled a pool with water from the softener, the water would be hungry for calcium and drag it out of the pool walls, making them feel and look like sandpaper. Calcium hardness should be kept within 200-275ppm. best to use regular water for topup and keep the pool water properly balanced (see your local pool store).
Probably the least expensive is to replace some or all of the water(dependind on hardness) with fresh.
Baking soda will not soften (remove calcium hardness) from pool water. It will raise total alkalinity.
Chemicals commonly used to treat hard water in pools include calcium hardness reducers or sequestrants. These products help to prevent the build-up of scale caused by hard water minerals like calcium and magnesium. It's important to follow the manufacturer's instructions and consult with a professional to determine the appropriate treatment for your specific pool.
Test your water for TDS and calcium hardness to decide if water needs to be replaced.
The best way to lower calcium hardness in the pool water is to drain the water and refill with water of lower calcium hardness. Have the calcium hardness tested for both the pool water and fill water. Ideal range is 175-300ppm. There are 'calcium reducer' products available through pool stores, but not sure how effective or economical they are.