Some dams, such as Lake Oroville in California, do use an afterbay to allow for off-peak water pumping back into the lake (https://norcalwater.org/efficient-water-management/efficient...).
Sadly, dams tend to destroy healthy fishery ecosystems —- a side effect of not “wasting” the water.
For one thing, existing dams absolutely cannot be converted to pumped hydro. Dams do not store water below them. Water flows downstream because a dam is in a river. There is no water to pump uphill. Unless, of course, you also build a second dam very close downstream to create another lower reservoir. This is usually a bad idea, and better to just find better geography that will support a new pumped hydro dam.