You could just build a back-channel for the existing hydro-dams? Those reservoirs are only full for a short period and that is when you dont need pump energy.

But where? In Finland, at least, the land is relatively flat when compared with Norway and Sweden, and with a large rural population there aren't really any good locations.

In my local area, we had major flooding this spring because the hydro plant operators were sleeping on the job (or whatever they did instead of regulating water levels). And that was a simple 2m increase in water levels.

NO/SE have some more geographically suitable locations, but last time I checked, flooding them was considered too environmentally destructive too the local environment.