FC will eventually overtake whatever is in the water if you add enough of it.
The danger is we can't say how much that will be because we don't know what's in the water. Something is there that is consuming FC. It's also being consumed by the FC, but we don't know how much of it is there.
So - you can either follow the protocol for Ammonia (which is to keep adding 10 FC at a time and retesting, repeat until FC holds) or you can exchange the water out. I have to imagine that the costs for the water exchange might be less, depending on what it costs to fill the pool. If you do drain, I would do a No-Drain Water Exchange to avoid potentially shifting the liner. Here's info on that:
Draining - Further Reading .
Which way to go depends on your cost of water and how lucky you feel. It could be the next 10ppm you put in finishes off what's in there and you hold chlorine fine. Or it could be dozens and dozens of chlorine jugs. Changing out the water is going to be more certain of a cost you can know, but will cost more than adding jugs if the next one would have been the last. Ultimately it's your call on which way to go.