One option is to use a water softener to treat the fill water. However, you would need to refill the pool first with treated water.
You could also plumb a water softener into the pool plumbing with a bypass so a small amount of water constantly goes through the softener. With adjustment valves, it could be set up such that you can get a fairly fast drop initially (i.e. divert a lot of water into the softner), but long term, it could be setup to maintain a 300-400 ppm level (i.e. divert less water into the softner). Since your fill water has a very high CH, you will probably need to divert at least a portion of the water traveling through the plumbing.
Either way it is probably an expensive solution but you may not have much of a choice because of the constant addition of CH from the fill water.
My fill water is about 200 ppm and I need to refill my pool every year as it climbs to over 800 ppm. I have thought about doing this myself but the city is bringing osmosis online this year to help with the mineral content so I want to see how well that works first.