Because you'd have to have engineered the whole thing for that purpose right from the get go. In theory you can run the generators in reverse and push water up the hill into the basin. In practice this may not work for a multitude of reasons (priming, encasement, rotation reversal, cavitation, impeller and impeller housing design).

Spain does have few (probably 4 according to ESP Wikipedia) pumped-storage hydroelectricity plants. Supposedly these are being used nowadays to store excess of energy produced by fotovoltaic plants. No idea how fast these can switch from storing energy to producing it and if these were used to help during the blackout.

Reconfiguring dams for storage isn't easy, quick or possible.

Non-storage dams don't have lower lakes to pull from, and the surrounding area might not be able to support it.

plus they also need water when there are droughts, which spain is also prone to.

they do it all the time