I've experimented a bit with adding some bicarbonate to very acidic espressos. It mutes the flavor a little bit but completely kills the harshness that made it unpleasant. It is not a fix for under-extraction though, it fixes beans that are just not good for espresso.

A tiny bit of salt works wonders for very bitter espressos from overly roasted beans.

But on the end buying beans I like is easier and makes even better coffee than those hacks.